MERGE Statement in SQL Explained

Prerequisite – MERGE Statement
As MERGE statement in SQL, as discussed before in the previous post, is the combination of three INSERT, DELETE and UPDATE statements. So if there is a Source table and a Target table that are to be merged, then with the help of MERGE statement, all the three operations (INSERT, UPDATE, DELETE) can be performed at once.

A simple example will clarify the use of 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.

Note: Refer this post for the syntax of MERGE statement.

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

SQL

/* 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 all we can perform all these three main statements in SQL together with the help of 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.