MERGE

Introduction#

MERGE (often also called UPSERT for “update or insert”) allows to insert new rows or, if a row already exists, to update the existing row. The point is to perform the whole set of operations atomically (to guarantee that the data remain consistent), and to prevent communication overhead for multiple SQL statements in a client/server system.

MERGE to make Target match Source

MERGE INTO targetTable t
    USING sourceTable s
        ON t.PKID = s.PKID
    WHEN MATCHED AND NOT EXISTS (
            SELECT s.ColumnA, s.ColumnB, s.ColumnC
            INTERSECT
            SELECT t.ColumnA, t.ColumnB, s.ColumnC
            )
        THEN UPDATE SET
            t.ColumnA = s.ColumnA
            ,t.ColumnB = s.ColumnB
            ,t.ColumnC = s.ColumnC
    WHEN NOT MATCHED BY TARGET
        THEN INSERT (PKID, ColumnA, ColumnB, ColumnC)
        VALUES (s.PKID, s.ColumnA, s.ColumnB, s.ColumnC)
    WHEN NOT MATCHED BY SOURCE
        THEN DELETE
    ;

Note: The AND NOT EXISTS portion prevents updating records that haven’t changed. Using the INTERSECT construct allows nullable columns to be compared without special handling.

MySQL: counting users by name

PostgreSQL: counting users by name