SQL MERGE Statement

SQL MERGE Statement combines INSERT, DELETE, and UPDATE statements into one single query.

MERGE Statement in SQL

MERGE statement in SQL is used to perform insert, update, and delete operations on a target table based on the results of JOIN with a source table. This allows users to synchronize two tables by performing operations on one table based on results from the second table.

The MERGE statement compares data between a source table and a target table based on specified key fields. It performs appropriate actions like inserting new records, updating existing ones, and deleting or flagging records no longer present in the source.

This statement provides a flexible way to manage data changes and is commonly used in scenarios like maintaining Slowly Changing Dimensions (SCD) in data warehouses.

Syntax

MERGE INTO target_table
USING source_table
ON merge_condition
WHEN MATCHED THEN
   UPDATE SET column1 = value1 [, column2 = value2 ...]
WHEN NOT MATCHED THEN
   INSERT (column1 [, column2 ...])
   VALUES (value1 [, value2 ...]);
 

SQL MERGE Statement Example

Suppose there are two tables: 

• PRODUCT_LIST which is the table that contains the current details about the products available with fields P_ID, P_NAME, and P_PRICE corresponding to the ID, name, and price of each product.
• UPDATED_LIST which is the table that contains the new details about the products available with fields P_ID, P_NAME, and P_PRICE corresponding to the ID, name, and price of each product.

The task is to update the details of the products in the PRODUCT_LIST as per the UPDATED_LIST.

Solution 

Now in order to explain this example better, let's split the example into steps. 

Step 1: Recognise the TARGET and the SOURCE table 

So in this example, since it is asked to update the products in the PRODUCT_LIST as per the UPDATED_LIST, hence the PRODUCT_LIST will act as the TARGET and UPDATED_LIST will act as the SOURCE table.

Step 2: Recognise the operations to be performed. 

Now as it can be seen that there are three mismatches between the TARGET and the SOURCE table, which are: 

1. The cost of COFFEE in TARGET is 15.00 while in SOURCE it is 25.00

      PRODUCT_LIST
102     COFFEE    15.00

      UPDATED_LIST
102     COFFEE    25.00

2. There is no BISCUIT product in SOURCE but it is in TARGET

      PRODUCT_LIST
103     BISCUIT   20.00

3. There is no CHIPS product in TARGET but it is in SOURCE

      UPDATED_LIST
104     CHIPS     22.00

Therefore, three operations need to be done in the TARGET according to the above discrepancies. They are:

1. UPDATE operation

102     COFFEE    25.00

2. DELETE operation

103     BISCUIT   20.00

3. INSERT operation

104     CHIPS     22.00

Step 3: Write the SQL Query

The SQL query to perform the above-mentioned operations with the help of the MERGE statement is:

/* Selecting the Target and the Source */
MERGE PRODUCT_LIST AS TARGET
    USING UPDATE_LIST AS SOURCE 

    /* 1. Performing the UPDATE operation */

    /* If the P_ID is same, 
       check for change in P_NAME or P_PRICE */
    ON (TARGET.P_ID = SOURCE.P_ID)
    WHEN MATCHED 
         AND TARGET.P_NAME <> SOURCE.P_NAME 
         OR TARGET.P_PRICE <> SOURCE.P_PRICE

    /* Update the records in TARGET */
    THEN UPDATE 
         SET TARGET.P_NAME = SOURCE.P_NAME,
         TARGET.P_PRICE = SOURCE.P_PRICE
     
    /* 2. Performing the INSERT operation */

    /* When no records are matched with TARGET table 
       Then insert the records in the target table */
    WHEN NOT MATCHED BY TARGET 
    THEN INSERT (P_ID, P_NAME, P_PRICE)          
         VALUES (SOURCE.P_ID, SOURCE.P_NAME, SOURCE.P_PRICE)

    /* 3. Performing the DELETE operation */

    /* When no records are matched with SOURCE table 
       Then delete the records from the target table */
    WHEN NOT MATCHED BY SOURCE 
    THEN DELETE

/* END OF MERGE */

Output:

  PRODUCT_LIST
P_ID    P_NAME    P_PRICE
101     TEA       10.00 
102     COFFEE    25.00
104     CHIPS     22.00

So, in this way, we can perform all these three main statements in SQL together with the help of the MERGE statement.

Note: Any name other than target and source can be used in the MERGE syntax. They are used only to give you a better explanation.

Important Points About SQL MERGE Statement

• The SQL MERGE statement combines INSERT, UPDATE, and DELETE operations into a single statement, allowing for efficient data synchronization between source and target tables.
• It provides flexibility in customizing complex SQL scripts by handling multiple data manipulation operations in a single transaction.
• The SQL MERGE statement is commonly used in scenarios like maintaining Slowly Changing Dimensions (SCD) in data warehouses.
• Proper indexing, optimized join conditions, and filtering the source table for necessary records can optimize the performance of the MERGE statement.