When using multiple WHEN MATCHED statements, do they all execute, or does only one get executed?

Question

If I have multiple WHEN MATCHED statements in a MERGE statement, do they all execute if they're true?

My example:

DECLARE @X bit = NULL;  --skipping the MERGE statement, straight to WHEN MATCHED  WHEN MATCHED AND A = 1     @X = 0; WHEN MATCHED AND B = 1     @X = 1; 

What is the state of X in each of the 4 possibilities?

A|B|X 0|0|? 0|1|? 1|0|? 1|1|? 

Basically, I'm curious if there's an implicit BREAK after each WHEN MATCHED clause.

Answer 1

I found in the MSDN documentation:

WHEN MATCHED THEN

Specifies that all rows of target_table that match the rows returned by ON , and satisfy any additional search condition, are either updated or deleted according to the clause.

The MERGE statement can have at most two WHEN MATCHED clauses. If two clauses are specified, then the first clause must be accompanied by an AND clause. For any given row, the second WHEN MATCHED clause is only applied if the first is not. If there are two WHEN MATCHED clauses, then one must specify an UPDATE action and one must specify a DELETE action. If UPDATE is specified in the clause, and more than one row of matches a row in target_table based on , SQL Server returns an error. The MERGE statement cannot update the same row more than once, or update and delete the same row.

So it looks like only one of the statements are executed, and they require a DELETE in one and an UPDATE in the other.

Answer 2

To answer your question, yes, it will only run a single match and then break. However, if you'd like to have logic to allow for conditional matching in the update, the CASE statement is rather useful for this.

Something like this as an example:

MERGE INTO YourTable USING (VALUES (1, 1, NULL), (0, 0, NULL), (0, 1, NULL), (1, 0, NULL))        T2 (a2,b2,c2) ON a = a2 AND b = b2 WHEN MATCHED  THEN     UPDATE SET c =        CASE          WHEN a = 1 THEN 0         WHEN b = 1 THEN 1         ELSE NULL       END         WHEN NOT MATCHED THEN     INSERT (a, b) VALUES (a2, b2);  SELECT * FROM YourTable ORDER BY a,b; 

• SQL Fiddle Demo

---

And the results:

A   B   C -------------- 0   0   (null) 0   1   1 1   0   0 1   1   0