SET vs. SELECT in T-SQL: Which Variable Assignment Method Should You Choose?

T-SQL variable assignment: comparison of SET and SELECT

In T-SQL, it can be confusing to choose SET or SELECT for variable assignment. This article will clarify the differences between these two methods and provide valuable guidance to developers.

Core Differences

• ANSI standard: SET complies with the ANSI variable assignment standard, while SELECT does not.
• Multiple assignment: SET handles a single variable assignment, while SELECT can perform multiple assignments at the same time.
• Query-based assignment: SET limits query results to scalar values ​​and will report an error when multiple values/rows are encountered. In contrast, SELECT assigns a return value to a variable, hiding the existence of other values.
• NULL processing: If there is no return value, SET assigns the value to NULL, while SELECT skips the assignment and retains the existing value of the variable.
• Speed ​​optimization: There is no inherent speed advantage between SET and SELECT. However, SELECT's ability to handle multiple assignments at once gives it a slight performance advantage.

Therefore, developers should consider the following factors when choosing an approach:

• Consistency: For ANSI-compliant assignments, use SET; for non-standard scenarios, use SELECT.
• Multiple assignment: When dealing with multiple variable assignments, in order to improve efficiency, use SELECT.
• Scalar query: For scalar query results, use SET.
• Robustness: To manage potential query result inconsistencies (e.g., missing values), it is recommended to use SELECT.
• Optimization: Take advantage of the speed of SELECT when necessary.

Mastering these subtle differences can enable developers to optimize variable assignments in T-SQL and improve code performance and robustness.

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