SET vs. SELECT in T-SQL: When Should I Use Each for Variable Assignment?

SET and SELECT statements in T-SQL: Comparison of variable assignment methods

In T-SQL, SET and SELECT statements serve different purposes when assigning values ​​to variables. Understanding their differences is crucial to writing efficient and error-free code.

SET: Standard variable assignment

The SET statement is a method in the ANSI standard for assigning a value to a single variable. Its syntax is concise and clear:

<code class="language-sql">SET variable_name = value;</code>

SELECT: Multi-variable assignment

Unlike SET, SELECT allows multiple variables to be assigned values ​​in a single statement. This is achieved by using the INTO keyword:

<code class="language-sql">SELECT column_1, column_2, ... INTO variable_1, variable_2, ...
FROM table_name
WHERE ...;</code>

Main differences

1. ANSI Compatibility: SET is an ANSI-compatible variable assignment method, while SELECT is not.
2. Single assignment and multiple assignment: SET can only assign values ​​to one variable at a time, while SELECT allows assigning values ​​to multiple variables at the same time.
3. Query assignment: When assigning from a query, SET only accepts scalar values ​​(single row and single column). And SELECT can retrieve multiple records/rows and assign them to multiple variables.
4. Null value handling: If the query does not return a value, SET will assign NULL to the variable, and SELECT will skip the assignment.
5. Speed: For single variable assignments, there is no significant performance difference between SET and SELECT. However, SELECT's ability to do multiple assignments simultaneously can provide a slight performance advantage.

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