MERGE - SQL Server to Aurora MySQL Migration Playbook
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MERGE

Feature compatibility Amazon SCT / Amazon DMS automation level Amazon SCT action code index Key differences


                              Three star feature compatibility


                              No automation

MERGE

Rewrite to use REPLACE and ON DUPLICATE KEY, or individual constituent DML statements.

SQL Server Usage

MERGE is a complex , hybrid DML/DQL statement for performing INSERT, UPDATE, or DELETE operations on a target table based on the results of a logical join of the target table and a source data set.

MERGE can also return row sets similar to SELECT using the OUTPUT clause, which gives the calling scope access to the actual data modifications of the MERGE statement.

The MERGE statement is most efficient for non-trivial conditional DML. For example, inserting data if a row key value doesn’t exist and updating the existing row if the key value already exists.

You can manage additional logic such as deleting rows from the target that don’t appear in the source. For simple, straightforward updates of data in one table based on data in another, it is typically more efficient to use simple INSERT, DELETE, and UPDATE statements. You can replicate all MERGE functionality using INSERT, DELETE, and UPDATE statements, but not necessarily less efficiently.

The SQL Server MERGE statement offers a wide range of functionality and flexibility and is compatible with ANSI standard SQL:2008. SQL Server has many extensions to MERGE that provide efficient T-SQL solutions for synchronizing data.

Syntax

MERGE [INTO] <Target Table> [AS] <Table Alias>]
USING <Source Table>
ON <Merge Predicate>
[WHEN MATCHED [AND <Predicate>]
THEN UPDATE SET <Column Assignments...> | DELETE]
[WHEN NOT MATCHED [BY TARGET] [AND <Predicate>]
THEN INSERT [(<Column List>)]
VALUES (<Values List>) | DEFAULT VALUES]
[WHEN NOT MATCHED BY SOURCE [AND <Predicate>]
THEN UPDATE SET <Column Assignments...> | DELETE]
OUTPUT [<Output Clause>]

Examples

Perform a simple one-way synchronization of two tables.

CREATE TABLE SourceTable
(
    Col1 INT NOT NULL PRIMARY KEY,
    Col2 VARCHAR(20) NOT NULL
);
CREATE TABLE TargetTable
(
    Col1 INT NOT NULL PRIMARY KEY,
    Col2 VARCHAR(20) NOT NULL
);
INSERT INTO SourceTable (Col1, Col2)
VALUES
(2, 'Source2'),
(3, 'Source3'),
(4, 'Source4');
INSERT INTO TargetTable (Col1, Col2)
VALUES
(1, 'Target1'),
(2, 'Target2'),
(3, 'Target3');
MERGE INTO TargetTable AS TGT
USING SourceTable AS SRC ON TGT.Col1 = SRC.Col1
WHEN MATCHED
    THEN UPDATE SET TGT.Col2 = SRC.Col2
WHEN NOT MATCHED
    THEN INSERT (Col1, Col2)
    VALUES (SRC.Col1, SRC.Col2);
SELECT * FROM TargetTable;
Col1  Col2
1     Target1
2     Source2
3     Source3
4     Source4

Perform a conditional two-way synchronization using NULL for no change and DELETE from the target when the data isn’t found in the source.

TRUNCATE TABLE SourceTable;
INSERT INTO SourceTable (Col1, Col2) VALUES (3, NULL), (4, 'NewSource4'), (5, 'Source5');
MERGE INTO TargetTable AS TGT
USING SourceTable AS SRC ON TGT.Col1 = SRC.Col1
WHEN MATCHED AND SRC.Col2 IS NOT NULL
    THEN UPDATE SET TGT.Col2 = SRC.Col2
WHEN NOT MATCHED
    THEN INSERT (Col1, Col2)
    VALUES (SRC.Col1, SRC.Col2)
WHEN NOT MATCHED BY SOURCE
    THEN DELETE;
SELECT *
FROM TargetTable;
Col1  Col2
3     Source3
4     NewSource4
5     Source5

For more information, see MERGE (Transact-SQL) in the SQL Server documentation.

MySQL Usage

Amazon Aurora MySQL-Compatible Edition (Aurora MySQL) doesn’t support the MERGE statement. However, it provides two other statements for merging data: REPLACE, and INSERT…​ ON DUPLICATE KEY UPDATE.

REPLACE deletes a row and inserts a new row if a duplicate key conflict occurs. INSERT…​ ON DUPLICATE KEY UPDATE performs an in-place update. Both REPLACE and ON DUPLICATE KEY UPDATE rely on an existing primary key and unique constraints. It isn’t possible to define custom MATCH conditions as with SQL Server MERGE statement.

REPLACE

REPLACE provides a function similar to INSERT. The difference is that REPLACE first deletes an existing row if a duplicate key violation for a PRIMARY KEY or UNIQUE constraint occurs.

REPLACE is a MySQL extension that isn’t ANSI compliant. It either performs only an INSERT when no duplicate key violations occur, or it performs a DELETE and then an INSERT if violations occur.

Syntax

REPLACE [INTO] <Table Name> (<Column List>)
VALUES (<Values List>)
REPLACE [INTO] <Table Name>
SET <Assignment List: ColumnName = VALUE...>
REPLACE [INTO] <Table Name> (<Column List>)
SELECT ...

INSERT …​ ON DUPLICATE KEY UPDATE

The ON DUPLICATE KEY UPDATE clause of the INSERT statement acts as a dual DML hybrid. Similar to REPLACE, it runs the assignments in the SET clause instead of raising a duplicate key error. ON DUPLICATE KEY UPDATE is a MySQL extension that in not ANSI compliant.

Syntax

INSERT [INTO] <Table Name> [<Column List>]
VALUES (<Value List>
ON DUPLICATE KEY <Assignment List: ColumnName = Value...>
INSERT [INTO] <Table Name>
SET <Assignment List: ColumnName = Value...>
ON DUPLICATE KEY
    UPDATE <Assignment List: ColumnName = Value...>
INSERT [INTO] <Table Name> [<Column List>]
SELECT ...
ON DUPLICATE KEY
    UPDATE <Assignment List: ColumnName = Value...>

Migration Considerations

REPLACE and INSERT …​ ON DUPLICATE KEY UPDATE don’t provide a full functional replacement for MERGE in SQL Server. The key differences are:

  • Key violation conditions are mandated by the primary key or unique constraints that exist on the target table. They can’t be defined using an explicit predicate.

  • There is no alternative for the WHEN NOT MATCHED BY SOURCE clause.

  • There is no alternative for the OUTPUT clause.

The key difference between REPLACE and INSERT ON DUPLICATE KEY UPDATE is that with REPLACE, the violating row is deleted or attempted to be deleted. If foreign keys are in place, the DELETE operation may fail, which may fail the entire transaction.

For INSERT …​ ON DUPLICATE KEY UPDATE, the update is performed on the existing row in place without attempting to delete it.

It should be straightforward to replace most MERGE statements with either REPLACE or INSERT…​ ON DUPLICATE KEY UPDATE.

Alternatively, break down the operations into their constituent INSERT, UPDATE, and DELETE statements.

Examples

Use REPLACE to create a simple one-way, two-table sync.

CREATE TABLE SourceTable
(
    Col1 INT NOT NULL PRIMARY KEY,
    Col2 VARCHAR(20) NOT NULL
);
CREATE TABLE TargetTable
(
    Col1 INT NOT NULL PRIMARY KEY,
    Col2 VARCHAR(20) NOT NULL
);
INSERT INTO SourceTable (Col1, Col2)
VALUES
(2, 'Source2'),
(3, 'Source3'),
(4, 'Source4');
INSERT INTO TargetTable (Col1, Col2)
VALUES
(1, 'Target1'),
(2, 'Target2'),
(3, 'Target3');
REPLACE INTO TargetTable(Col1, Col2)
SELECT Col1,
    Col2
FROM SourceTable;
SELECT *
FROM TargetTable;
Col1  Col2
1     Target1
2     Source2
3     Source3
4     Source4

Create a conditional two-way sync using NULL for no change and DELETE from target when not found in source.

TRUNCATE TABLE SourceTable;
INSERT INTO SourceTable(Col1, Col2)
VALUES
(3, NULL),
(4, 'NewSource4'),
(5, 'Source5');
DELETE FROM TargetTable
WHERE Col1 NOT IN (SELECT Col1 FROM SourceTable);
INSERT INTO TargetTable (Col1, Col2)
SELECT Col1,
    Col2
FROM SourceTable AS SRC
WHERE SRC.Col1 NOT IN (
    SELECT Col1
    FROM TargetTable
);
UPDATE TargetTable AS TGT
SET Col2 = (
    SELECT COALESCE(SRC.Col2, TGT.Col2)
    FROM SourceTable AS SRC
    WHERE SRC.Col1 = TGT.Col1
    )
WHERE TGT.Col1 IN (
    SELECT Col1
    FROM SourceTable
);
SELECT *
FROM TargetTable;
Col1  Col2
3     Source3
4     NewSource4
5     Source5

Summary

The following table describes similarities, differences, and key migration considerations.

SQL Server MERGE feature Migrate to Aurora MySQL Comments

Define source set in USING clause.

Define source set in a SELECT query or in a table.

Define logical duplicate key condition with an ON predicate.

Duplicate key condition mandated by primary key and unique constraints on target table.

WHEN MATCHED THEN UPDATE

REPLACE or INSERT…​ ON DUPLICATE KEY UPDATE

When using REPLACE, the violating row will be deleted, or attempted to be deleted. If there are foreign keys in place, the DELETE operation may fail, which may fail the entire transaction.

With INSERT …​ ON DUPLICATE KEY UPDATE, the updated is performed on the existing row in place, without attempting to delete it.

WHEN MATCHED THEN DELETE

DELETE FROM Target WHERE Key IN (SELECT Key FROM Source)

WHEN NOT MATCHED THEN INSERT

REPLACE or INSERT…​ ON DUPLICATE KEY UPDATE

See the preceding comment.

WHEN NOT MATCHED BY SOURCE UPDATE

UPDATE Target SET <assignments> WHERE Key NOT IN (SELECT Key FROM Source)

WHEN NOT MATCHED BY SOURCE DELETE

DELETE FROM Target WHERE KEY NOT IN (SELECT Key FROM Source)

OUTPUT clause

N/A

For more information, see REPLACE Statement and INSERT …​ ON DUPLICATE KEY UPDATE Statement in the MySQL documentation.