Microsoft Sql Server
2021年11月18日Download here: http://gg.gg/wxck9
*Mar 02, 2021 The Microsoft JDBC Driver for SQL Server is a Type 4 JDBC driver that provides database connectivity through the standard JDBC application program interfaces (APIs) available on the Java platform. The driver downloads are available to all users at no extra charge.
*Bring Microsoft SQL Server 2017 to the platform of your choice. Use SQL Server 2017 on Windows, Linux, and Docker containers.
*Microsoft Sql Server Online
*Microsoft Sql Server Vss
*Microsoft Sql Server Error 18456
*Microsoft Sql Server Management Studio
Microsoft SQL server training accreditations can be very beneficial and also useful for some companies, employers, as well as to some SQL professionals directly. According to several surveys, 35% of respondents experienced a pay rise post-certification, and 48% think Microsoft SQL Server qualifications are handy in improving Work possibilities.-->
Applies to: SQL Server (all supported versions) Azure SQL Database Azure Synapse Analytics
Runs insert, update, or delete operations on a target table from the results of a join with a source table. For example, synchronize two tables by inserting, updating, or deleting rows in one table based on differences found in the other table.
Note
MERGE is currently in preview for Azure Synapse Analytics.
Performance Tip: The conditional behavior described for the MERGE statement works best when the two tables have a complex mixture of matching characteristics. For example, inserting a row if it doesn’t exist, or updating a row if it matches. When simply updating one table based on the rows of another table, improve the performance and scalability with basic INSERT, UPDATE, and DELETE statements. For example:Syntax
Note
To view Transact-SQL syntax for SQL Server 2014 and earlier, see Previous versions documentation.Arguments
WITH <common_table_expression>Specifies the temporary named result set or view, also known as common table expression, that’s defined within the scope of the MERGE statement. The result set derives from a simple query and is referenced by the MERGE statement. For more information, see WITH common_table_expression (Transact-SQL).
TOP ( expression ) [ PERCENT ]Specifies the number or percentage of affected rows. expression can be either a number or a percentage of the rows. The rows referenced in the TOP expression are not arranged in any order. For more information, see TOP (Transact-SQL).
The TOP clause applies after the entire source table and the entire target table join and the joined rows that don’t qualify for an insert, update, or delete action are removed. The TOP clause further reduces the number of joined rows to the specified value. The insert, update, or delete actions apply to the remaining joined rows in an unordered way. That is, there’s no order in which the rows are distributed among the actions defined in the WHEN clauses. For example, specifying TOP (10) affects 10 rows. Of these rows, 7 may be updated and 3 inserted, or 1 may be deleted, 5 updated, and 4 inserted, and so on.
Because the MERGE statement does a full table scan of both the source and target tables, I/O performance is sometimes affected when using the TOP clause to modify a large table by creating multiple batches. In this scenario, it’s important to ensure that all successive batches target new rows.
database_nameThe name of the database in which target_table is located.
schema_nameThe name of the schema to which target_table belongs.
target_tableThe table or view against which the data rows from <table_source> are matched based on <clause_search_condition>. target_table is the target of any insert, update, or delete operations specified by the WHEN clauses of the MERGE statement.
If target_table is a view, any actions against it must satisfy the conditions for updating views. For more information, see Modify Data Through a View.
target_table can’t be a remote table. target_table can’t have any rules defined on it.
[ AS ] table_aliasAn alternative name to reference a table for the target_table.
USING <table_source>Specifies the data source that’s matched with the data rows in target_table based on <merge_search condition>. The result of this match dictates the actions to take by the WHEN clauses of the MERGE statement. <table_source> can be a remote table or a derived table that accesses remote tables.
<table_source> can be a derived table that uses the Transact-SQL table value constructor to construct a table by specifying multiple rows.
[ AS ] table_aliasAn alternative name to reference a table for the table_source.
For more information about the syntax and arguments of this clause, see FROM (Transact-SQL).
ON <merge_search_condition>Specifies the conditions on which <table_source> joins with target_table to determine where they match.
Caution
It’s important to specify only the columns from the target table to use for matching purposes. That is, specify columns from the target table that are compared to the corresponding column of the source table. Don’t attempt to improve query performance by filtering out rows in the target table in the ON clause; for example, such as specifying AND NOT target_table.column_x = value. Doing so may return unexpected and incorrect results.
WHEN MATCHED THEN <merge_matched>Specifies that all rows of *target_table, which match the rows returned by <table_source> ON <merge_search_condition>, and satisfy any additional search condition, are either updated or deleted according to the <merge_matched> clause.
The MERGE statement can have, at most, two WHEN MATCHED clauses. If two clauses are specified, the first clause must be accompanied by an AND <search_condition> clause. For any given row, the second WHEN MATCHED clause is only applied if the first isn’t. If there are two WHEN MATCHED clauses, one must specify an UPDATE action and one must specify a DELETE action. When UPDATE is specified in the <merge_matched> clause, and more than one row of <table_source> matches a row in target_table based on <merge_search_condition>, SQL Server returns an error. The MERGE statement can’t update the same row more than once, or update and delete the same row.
WHEN NOT MATCHED [ BY TARGET ] THEN <merge_not_matched>Specifies that a row is inserted into target_table for every row returned by <table_source> ON <merge_search_condition> that doesn’t match a row in target_table, but satisfies an additional search condition, if present. The values to insert are specified by the <merge_not_matched> clause. The MERGE statement can have only one WHEN NOT MATCHED [ BY TARGET ] clause.
WHEN NOT MATCHED BY SOURCE THEN <merge_matched>Specifies that all rows of *target_table, which don’t match the rows returned by <table_source> ON <merge_search_condition>, and that satisfy any additional search condition, are updated or deleted according to the <merge_matched> clause.
The MERGE statement can have at most two WHEN NOT MATCHED BY SOURCE clauses. If two clauses are specified, then the first clause must be accompanied by an AND <clause_search_condition> clause. For any given row, the second WHEN NOT MATCHED BY SOURCE clause is only applied if the first isn’t. If there are two WHEN NOT MATCHED BY SOURCE clauses, then one must specify an UPDATE action and one must specify a DELETE action. Only columns from the target table can be referenced in <clause_search_condition>.
When no rows are returned by <table_source>, columns in the source table can’t be accessed. If the update or delete action specified in the <merge_matched> clause references columns in the source table, error 207 (Invalid column name) is returned. For example, the clause WHEN NOT MATCHED BY SOURCE THEN UPDATE SET TargetTable.Col1 = SourceTable.Col1 may cause the statement to fail because Col1 in the source table is inaccessible.
AND <clause_search_condition>Specifies any valid search condition. For more information, see Search Condition (Transact-SQL).
<table_hint_limited>Specifies one or more table hints to apply on the target table for each of the insert, update, or delete actions done by the MERGE statement. The WITH keyword and the parentheses are required.
NOLOCK and READUNCOMMITTED aren’t allowed. For more information about table hints, see Table Hints (Transact-SQL).
Specifying the TABLOCK hint on a table that’s the target of an INSERT statement has the same effect as specifying the TABLOCKX hint. An exclusive lock is taken on the table. When FORCESEEK is specified, it applies to the implicit instance of the target table joined with the source table.
Caution
Specifying READPAST with WHEN NOT MATCHED [ BY TARGET ] THEN INSERT may result in INSERT operations that violate UNIQUE constraints.
INDEX ( index_val [ ,...n ] )Specifies the name or ID of one or more indexes on the target table for doing an implicit join with the source table. For more information, see Table Hints (Transact-SQL).
<output_clause>Returns a row for every row in target_table that’s updated, inserted, or deleted, in no particular order. $action can be specified in the output clause. $action is a column of type nvarchar(10) that returns one of three values for each row: ’INSERT’, ’UPDATE’, or ’DELETE’, according to the action done on that row. For more information about the arguments and behavior of this clause, see OUTPUT Clause (Transact-SQL).
OPTION ( <query_hint> [ ,...n ] )Specifies that optimizer hints are used to customize the way the Database Engine processes the statement. For more information, see Query Hints (Transact-SQL).
<merge_matched>Specifies the update or delete action that’s applied to all rows of target_table that don’t match the rows returned by <table_source> ON <merge_search_condition>, and which satisfy any additional search condition.
UPDATE SET <set_clause>Specifies the list of column or variable names to update in the target table and the values with which to update them.
For more information about the arguments of this clause, see UPDATE (Transact-SQL). Setting a variable to the same value as a column isn’t supported.
DELETESpecifies that the rows matching rows in target_table are deleted.
<merge_not_matched>Specifies the values to insert into the target table.
(column_list)A list of one or more columns of the target table in which to insert data. Columns must be specified as a single-part name or else the MERGE statement will fail. column_list must be enclosed in parentheses and delimited by commas.
VALUES ( values_list)A comma-separated list of constants, variables, or expressions that return values to insert into the target table. Expressions can’t contain an EXECUTE statement.
DEFAULT VALUESForces the inserted row to contain the default values defined for each column.
For more information about this clause, see INSERT (Transact-SQL).
<search_condition>Specifies the search conditions to specify <merge_search_condition> or <clause_search_condition>. For more information about the arguments for this clause, see Search Condition (Transact-SQL).
<graph search pattern>Specifies the graph match pattern. For more information about the arguments for this clause, see MATCH (Transact-SQL)Remarks
Note
In Azure Synapse Analytics, the MERGE command (preview) has following differences compared to SQL server and Azure SQL database.
*A MERGE update is implemented as a delete and insert pair. The affected row count for a MERGE update includes the deleted and inserted rows.
*During the preview, MERGE…WHEN NOT MATCHED INSERT is not supported for tables with IDENTITY columns.
*The support for tables with different distribution types is described in this table:Microsoft Sql Server OnlineMERGE CLAUSE in Azure Synapse AnalyticsSupported TARGE distribution tableSupported SOURCE distribution tableCommentWHEN MATCHEDAll distribution typesAll distribution typesNOT MATCHED BY TARGETHASHAll distribution typesUse UPDATE/DELETE FROM…JOIN to synchronize two tables.NOT MATCHED BY SOURCEAll distribution typesAll distribution types
Important
Preview features are meant for testing only and should not be used on production instances or production data. Please also keep a copy of your test data if the data is important.
In Azure Synapse Analytics the MERGE command, currently in preview, may, under certain conditions, leave the target table in an inconsistent state, with rows placed in the wrong distribution, causing later queries to return wrong results in some cases. This problem may happen when these two conditions are met:
*The MERGE T-SQL statement was executed on a HASH distributed TARGET table in Azure Synapse SQL database AND
*The TARGET table of the merge has secondary indices or a UNIQUE constraint.
The problem has been fixed in Synapse SQL version 10.0.15563.0 and higher.
*To check, connect to the Synapse SQL database via SQL Server Management Studio (SSMS) and run SELECT @@VERSION. If the fix has not been applied, manually pause and resume your Synapse SQL pool to get the fix.
*Until the fix has been verified applied to your Synapse SQL pool, avoid using the MERGE command on HASH distributed TARGET tables that have secondary indices or UNIQUE constraints.
*This fix doesn’t repair tables already affected by the MERGE problem. Use scripts below to identify and repair any affected tables manually.
To check which hash distributed tables in a database cannot work with MERGE due to this issue, run this statement
To check if a hash distributed TARGET table for MERGE is affected by this issue, follow these steps to examine if the tables have rows landed in wrong distribution. If ’no need for repair’ is returned, this table is not affected.
To repair affected tables, run these statements to copy all rows from the old table to a new table.
At least one of the three MATCHED clauses must be specified, but they can be specified in any order. A variable can’t be updated more than once in the same MATCHED clause.
Any insert, update, or delete action specified on the target table by the MERGE statement are limited by any constraints defined on it, including any cascading referential integrity constraints. If IGNORE_DUP_KEY is ON for any unique indexes on the target table, MERGE ignores this setting.
The MERGE statement requires a semicolon (;) as a statement terminator. Error 10713 is raised when a MERGE statement is run without the terminator.
When used after MERGE, @@ROWCOUNT (Transact-SQL) returns the total number of rows inserted, updated, and deleted to the client.
MERGE is a fully reserved keyword when the database compatibility level is set to 100 or higher. The MERGE statement is available under both 90 and 100 database compatibility levels; however, the keyword isn’t fully reserved when the database compatibility level is set to 90.
Don’t use the MERGE statement when using queued updating replication. The MERGE and queued updating trigger aren’t compatible. Replace the MERGE statement with an insert or an update statement.Trigger implementation
For every insert, update, or delete action specified in the MERGE statement, SQL Server fires any corresponding AFTER triggers defined on the target table, but doesn’t guarantee on which action to fire triggers first or last. Triggers defined for the same action honor the order you specify. For more information about setting trigger firing order, see Specify First and Last Triggers.
If the target table has an enabled INSTEAD OF trigger defined on it for an insert, update, or delete action done by a MERGE statement, it must have an enabled INSTEAD OF trigger for all of the actions specified in the MERGE statement.
If any INSTEAD OF UPDATE or INSTEAD OF DELETE triggers are defined on target_table, the update or delete operations aren’t run. Instead, the triggers fire and the inserted and deleted tables then populate accordingly.
If any INSTEAD OF INSERT triggers are defined on target_table, the insert operation isn’t performed. Instead, the table populates accordingly.Permissions
Requires SELECT permission on the source table and INSERT, UPDATE, or DELETE permissions on the target table. For more information, see the Permissions section in the SELECT, INSERT, UPDATE, and DELETE articles.Optimizing MERGE statement performance
By using the MERGE statement, you can replace the individual DML statements with a single statement. This can improve query performance because the operations are performed within a single statement, therefore, minimizing the number of times the data in the source and target tables are processed. However, performance gains depend on having correct indexes, joins, and other considerations in place.Index best practices
To improve the performance of the MERGE statement, we recommend the following index guidelines:
*Create an index on the join columns in the source table that is unique and covering.
*Create a unique clustered index on the join columns in the target table.
These indexes ensure that the join keys are unique and the data in the tables is sorted. Query performance is improved because the query optimizer does not need to perform extra validation processing to locate and update duplicate rows and additional sort operations are not necessary.JOIN best practices
To improve the performance of the MERGE statement and ensure correct results are obtained, we recommend the following join guidelines:
*Specify only search conditions in the ON <merge_search_condition> clause that determine the criteria for matching data in the source and target tables. That is, specify only columns from the target table that are compared to the corresponding columns of the source table.
*Do not include comparisons to other values such as a constant.
To filter out rows from the source or target tables, use one of the following methods.
*Specify the search condition for row filtering in the appropriate WHEN clause. For example, WHEN NOT MATCHED AND S.EmployeeName LIKE ’S%’ THEN INSERT....
*Define a view on the source or target that returns the filtered rows and reference the view as the source or target table. If the view is defined on the target table, any actions against it must satisfy the conditions for updating views. For more information about updating data by using a view, see Modifying Data Through a View.
*Use the WITH <common table expression> clause to filter out rows from the source or target tables. This method is similar to specifying additional search criteria in the ON clause and may produce incorrect results. We recommend that you avoid using this method or test thoroughly before implementing it.
The join operation in the MERGE statement is optimized in the same way as a join in a SELECT statement. That is, when SQL Server processes joins, the query optimizer chooses the most efficient metho
https://diarynote-jp.indered.space
*Mar 02, 2021 The Microsoft JDBC Driver for SQL Server is a Type 4 JDBC driver that provides database connectivity through the standard JDBC application program interfaces (APIs) available on the Java platform. The driver downloads are available to all users at no extra charge.
*Bring Microsoft SQL Server 2017 to the platform of your choice. Use SQL Server 2017 on Windows, Linux, and Docker containers.
*Microsoft Sql Server Online
*Microsoft Sql Server Vss
*Microsoft Sql Server Error 18456
*Microsoft Sql Server Management Studio
Microsoft SQL server training accreditations can be very beneficial and also useful for some companies, employers, as well as to some SQL professionals directly. According to several surveys, 35% of respondents experienced a pay rise post-certification, and 48% think Microsoft SQL Server qualifications are handy in improving Work possibilities.-->
Applies to: SQL Server (all supported versions) Azure SQL Database Azure Synapse Analytics
Runs insert, update, or delete operations on a target table from the results of a join with a source table. For example, synchronize two tables by inserting, updating, or deleting rows in one table based on differences found in the other table.
Note
MERGE is currently in preview for Azure Synapse Analytics.
Performance Tip: The conditional behavior described for the MERGE statement works best when the two tables have a complex mixture of matching characteristics. For example, inserting a row if it doesn’t exist, or updating a row if it matches. When simply updating one table based on the rows of another table, improve the performance and scalability with basic INSERT, UPDATE, and DELETE statements. For example:Syntax
Note
To view Transact-SQL syntax for SQL Server 2014 and earlier, see Previous versions documentation.Arguments
WITH <common_table_expression>Specifies the temporary named result set or view, also known as common table expression, that’s defined within the scope of the MERGE statement. The result set derives from a simple query and is referenced by the MERGE statement. For more information, see WITH common_table_expression (Transact-SQL).
TOP ( expression ) [ PERCENT ]Specifies the number or percentage of affected rows. expression can be either a number or a percentage of the rows. The rows referenced in the TOP expression are not arranged in any order. For more information, see TOP (Transact-SQL).
The TOP clause applies after the entire source table and the entire target table join and the joined rows that don’t qualify for an insert, update, or delete action are removed. The TOP clause further reduces the number of joined rows to the specified value. The insert, update, or delete actions apply to the remaining joined rows in an unordered way. That is, there’s no order in which the rows are distributed among the actions defined in the WHEN clauses. For example, specifying TOP (10) affects 10 rows. Of these rows, 7 may be updated and 3 inserted, or 1 may be deleted, 5 updated, and 4 inserted, and so on.
Because the MERGE statement does a full table scan of both the source and target tables, I/O performance is sometimes affected when using the TOP clause to modify a large table by creating multiple batches. In this scenario, it’s important to ensure that all successive batches target new rows.
database_nameThe name of the database in which target_table is located.
schema_nameThe name of the schema to which target_table belongs.
target_tableThe table or view against which the data rows from <table_source> are matched based on <clause_search_condition>. target_table is the target of any insert, update, or delete operations specified by the WHEN clauses of the MERGE statement.
If target_table is a view, any actions against it must satisfy the conditions for updating views. For more information, see Modify Data Through a View.
target_table can’t be a remote table. target_table can’t have any rules defined on it.
[ AS ] table_aliasAn alternative name to reference a table for the target_table.
USING <table_source>Specifies the data source that’s matched with the data rows in target_table based on <merge_search condition>. The result of this match dictates the actions to take by the WHEN clauses of the MERGE statement. <table_source> can be a remote table or a derived table that accesses remote tables.
<table_source> can be a derived table that uses the Transact-SQL table value constructor to construct a table by specifying multiple rows.
[ AS ] table_aliasAn alternative name to reference a table for the table_source.
For more information about the syntax and arguments of this clause, see FROM (Transact-SQL).
ON <merge_search_condition>Specifies the conditions on which <table_source> joins with target_table to determine where they match.
Caution
It’s important to specify only the columns from the target table to use for matching purposes. That is, specify columns from the target table that are compared to the corresponding column of the source table. Don’t attempt to improve query performance by filtering out rows in the target table in the ON clause; for example, such as specifying AND NOT target_table.column_x = value. Doing so may return unexpected and incorrect results.
WHEN MATCHED THEN <merge_matched>Specifies that all rows of *target_table, which match the rows returned by <table_source> ON <merge_search_condition>, and satisfy any additional search condition, are either updated or deleted according to the <merge_matched> clause.
The MERGE statement can have, at most, two WHEN MATCHED clauses. If two clauses are specified, the first clause must be accompanied by an AND <search_condition> clause. For any given row, the second WHEN MATCHED clause is only applied if the first isn’t. If there are two WHEN MATCHED clauses, one must specify an UPDATE action and one must specify a DELETE action. When UPDATE is specified in the <merge_matched> clause, and more than one row of <table_source> matches a row in target_table based on <merge_search_condition>, SQL Server returns an error. The MERGE statement can’t update the same row more than once, or update and delete the same row.
WHEN NOT MATCHED [ BY TARGET ] THEN <merge_not_matched>Specifies that a row is inserted into target_table for every row returned by <table_source> ON <merge_search_condition> that doesn’t match a row in target_table, but satisfies an additional search condition, if present. The values to insert are specified by the <merge_not_matched> clause. The MERGE statement can have only one WHEN NOT MATCHED [ BY TARGET ] clause.
WHEN NOT MATCHED BY SOURCE THEN <merge_matched>Specifies that all rows of *target_table, which don’t match the rows returned by <table_source> ON <merge_search_condition>, and that satisfy any additional search condition, are updated or deleted according to the <merge_matched> clause.
The MERGE statement can have at most two WHEN NOT MATCHED BY SOURCE clauses. If two clauses are specified, then the first clause must be accompanied by an AND <clause_search_condition> clause. For any given row, the second WHEN NOT MATCHED BY SOURCE clause is only applied if the first isn’t. If there are two WHEN NOT MATCHED BY SOURCE clauses, then one must specify an UPDATE action and one must specify a DELETE action. Only columns from the target table can be referenced in <clause_search_condition>.
When no rows are returned by <table_source>, columns in the source table can’t be accessed. If the update or delete action specified in the <merge_matched> clause references columns in the source table, error 207 (Invalid column name) is returned. For example, the clause WHEN NOT MATCHED BY SOURCE THEN UPDATE SET TargetTable.Col1 = SourceTable.Col1 may cause the statement to fail because Col1 in the source table is inaccessible.
AND <clause_search_condition>Specifies any valid search condition. For more information, see Search Condition (Transact-SQL).
<table_hint_limited>Specifies one or more table hints to apply on the target table for each of the insert, update, or delete actions done by the MERGE statement. The WITH keyword and the parentheses are required.
NOLOCK and READUNCOMMITTED aren’t allowed. For more information about table hints, see Table Hints (Transact-SQL).
Specifying the TABLOCK hint on a table that’s the target of an INSERT statement has the same effect as specifying the TABLOCKX hint. An exclusive lock is taken on the table. When FORCESEEK is specified, it applies to the implicit instance of the target table joined with the source table.
Caution
Specifying READPAST with WHEN NOT MATCHED [ BY TARGET ] THEN INSERT may result in INSERT operations that violate UNIQUE constraints.
INDEX ( index_val [ ,...n ] )Specifies the name or ID of one or more indexes on the target table for doing an implicit join with the source table. For more information, see Table Hints (Transact-SQL).
<output_clause>Returns a row for every row in target_table that’s updated, inserted, or deleted, in no particular order. $action can be specified in the output clause. $action is a column of type nvarchar(10) that returns one of three values for each row: ’INSERT’, ’UPDATE’, or ’DELETE’, according to the action done on that row. For more information about the arguments and behavior of this clause, see OUTPUT Clause (Transact-SQL).
OPTION ( <query_hint> [ ,...n ] )Specifies that optimizer hints are used to customize the way the Database Engine processes the statement. For more information, see Query Hints (Transact-SQL).
<merge_matched>Specifies the update or delete action that’s applied to all rows of target_table that don’t match the rows returned by <table_source> ON <merge_search_condition>, and which satisfy any additional search condition.
UPDATE SET <set_clause>Specifies the list of column or variable names to update in the target table and the values with which to update them.
For more information about the arguments of this clause, see UPDATE (Transact-SQL). Setting a variable to the same value as a column isn’t supported.
DELETESpecifies that the rows matching rows in target_table are deleted.
<merge_not_matched>Specifies the values to insert into the target table.
(column_list)A list of one or more columns of the target table in which to insert data. Columns must be specified as a single-part name or else the MERGE statement will fail. column_list must be enclosed in parentheses and delimited by commas.
VALUES ( values_list)A comma-separated list of constants, variables, or expressions that return values to insert into the target table. Expressions can’t contain an EXECUTE statement.
DEFAULT VALUESForces the inserted row to contain the default values defined for each column.
For more information about this clause, see INSERT (Transact-SQL).
<search_condition>Specifies the search conditions to specify <merge_search_condition> or <clause_search_condition>. For more information about the arguments for this clause, see Search Condition (Transact-SQL).
<graph search pattern>Specifies the graph match pattern. For more information about the arguments for this clause, see MATCH (Transact-SQL)Remarks
Note
In Azure Synapse Analytics, the MERGE command (preview) has following differences compared to SQL server and Azure SQL database.
*A MERGE update is implemented as a delete and insert pair. The affected row count for a MERGE update includes the deleted and inserted rows.
*During the preview, MERGE…WHEN NOT MATCHED INSERT is not supported for tables with IDENTITY columns.
*The support for tables with different distribution types is described in this table:Microsoft Sql Server OnlineMERGE CLAUSE in Azure Synapse AnalyticsSupported TARGE distribution tableSupported SOURCE distribution tableCommentWHEN MATCHEDAll distribution typesAll distribution typesNOT MATCHED BY TARGETHASHAll distribution typesUse UPDATE/DELETE FROM…JOIN to synchronize two tables.NOT MATCHED BY SOURCEAll distribution typesAll distribution types
Important
Preview features are meant for testing only and should not be used on production instances or production data. Please also keep a copy of your test data if the data is important.
In Azure Synapse Analytics the MERGE command, currently in preview, may, under certain conditions, leave the target table in an inconsistent state, with rows placed in the wrong distribution, causing later queries to return wrong results in some cases. This problem may happen when these two conditions are met:
*The MERGE T-SQL statement was executed on a HASH distributed TARGET table in Azure Synapse SQL database AND
*The TARGET table of the merge has secondary indices or a UNIQUE constraint.
The problem has been fixed in Synapse SQL version 10.0.15563.0 and higher.
*To check, connect to the Synapse SQL database via SQL Server Management Studio (SSMS) and run SELECT @@VERSION. If the fix has not been applied, manually pause and resume your Synapse SQL pool to get the fix.
*Until the fix has been verified applied to your Synapse SQL pool, avoid using the MERGE command on HASH distributed TARGET tables that have secondary indices or UNIQUE constraints.
*This fix doesn’t repair tables already affected by the MERGE problem. Use scripts below to identify and repair any affected tables manually.
To check which hash distributed tables in a database cannot work with MERGE due to this issue, run this statement
To check if a hash distributed TARGET table for MERGE is affected by this issue, follow these steps to examine if the tables have rows landed in wrong distribution. If ’no need for repair’ is returned, this table is not affected.
To repair affected tables, run these statements to copy all rows from the old table to a new table.
At least one of the three MATCHED clauses must be specified, but they can be specified in any order. A variable can’t be updated more than once in the same MATCHED clause.
Any insert, update, or delete action specified on the target table by the MERGE statement are limited by any constraints defined on it, including any cascading referential integrity constraints. If IGNORE_DUP_KEY is ON for any unique indexes on the target table, MERGE ignores this setting.
The MERGE statement requires a semicolon (;) as a statement terminator. Error 10713 is raised when a MERGE statement is run without the terminator.
When used after MERGE, @@ROWCOUNT (Transact-SQL) returns the total number of rows inserted, updated, and deleted to the client.
MERGE is a fully reserved keyword when the database compatibility level is set to 100 or higher. The MERGE statement is available under both 90 and 100 database compatibility levels; however, the keyword isn’t fully reserved when the database compatibility level is set to 90.
Don’t use the MERGE statement when using queued updating replication. The MERGE and queued updating trigger aren’t compatible. Replace the MERGE statement with an insert or an update statement.Trigger implementation
For every insert, update, or delete action specified in the MERGE statement, SQL Server fires any corresponding AFTER triggers defined on the target table, but doesn’t guarantee on which action to fire triggers first or last. Triggers defined for the same action honor the order you specify. For more information about setting trigger firing order, see Specify First and Last Triggers.
If the target table has an enabled INSTEAD OF trigger defined on it for an insert, update, or delete action done by a MERGE statement, it must have an enabled INSTEAD OF trigger for all of the actions specified in the MERGE statement.
If any INSTEAD OF UPDATE or INSTEAD OF DELETE triggers are defined on target_table, the update or delete operations aren’t run. Instead, the triggers fire and the inserted and deleted tables then populate accordingly.
If any INSTEAD OF INSERT triggers are defined on target_table, the insert operation isn’t performed. Instead, the table populates accordingly.Permissions
Requires SELECT permission on the source table and INSERT, UPDATE, or DELETE permissions on the target table. For more information, see the Permissions section in the SELECT, INSERT, UPDATE, and DELETE articles.Optimizing MERGE statement performance
By using the MERGE statement, you can replace the individual DML statements with a single statement. This can improve query performance because the operations are performed within a single statement, therefore, minimizing the number of times the data in the source and target tables are processed. However, performance gains depend on having correct indexes, joins, and other considerations in place.Index best practices
To improve the performance of the MERGE statement, we recommend the following index guidelines:
*Create an index on the join columns in the source table that is unique and covering.
*Create a unique clustered index on the join columns in the target table.
These indexes ensure that the join keys are unique and the data in the tables is sorted. Query performance is improved because the query optimizer does not need to perform extra validation processing to locate and update duplicate rows and additional sort operations are not necessary.JOIN best practices
To improve the performance of the MERGE statement and ensure correct results are obtained, we recommend the following join guidelines:
*Specify only search conditions in the ON <merge_search_condition> clause that determine the criteria for matching data in the source and target tables. That is, specify only columns from the target table that are compared to the corresponding columns of the source table.
*Do not include comparisons to other values such as a constant.
To filter out rows from the source or target tables, use one of the following methods.
*Specify the search condition for row filtering in the appropriate WHEN clause. For example, WHEN NOT MATCHED AND S.EmployeeName LIKE ’S%’ THEN INSERT....
*Define a view on the source or target that returns the filtered rows and reference the view as the source or target table. If the view is defined on the target table, any actions against it must satisfy the conditions for updating views. For more information about updating data by using a view, see Modifying Data Through a View.
*Use the WITH <common table expression> clause to filter out rows from the source or target tables. This method is similar to specifying additional search criteria in the ON clause and may produce incorrect results. We recommend that you avoid using this method or test thoroughly before implementing it.
The join operation in the MERGE statement is optimized in the same way as a join in a SELECT statement. That is, when SQL Server processes joins, the query optimizer chooses the most efficient metho
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