这是对我之前的问题的跟进，在 t-sql 中
SELECT SCOPE_IDENTITY()
返回一个 BIGINT，我做了以下让它返回一个 INT：
DECLARE @X INT
INSERT ...
SELECT @X = SCOPE_IDENTITY()
-- if i don't include the line below, it will return a BIGINT
SELECT @X
为什么它会返回 BIGINT 除非我最后执行 SELECT @X ？
ps原来是
SELECT @X = SCOPE_IDENTITY()
不返回任何东西，它只是设置@x
The statement
SELECT @X = SCOPE_IDENTITY()
is an assignment statement. As in most programming languages, an assignment statement is executed by first evaluating the right hand side. In this case the right hand side evaluates to a bigint. When the value of @X gets the resulting bigint, there is an implicit type conversion, because @X is a different type (int) than the value it's receiving.
SQL is a typed language, and the type of an expression (such as SCOPE_IDENTITY() here) depends on the expression, not on what happens to the expression's value after evaluation.
Analogy:
DECLARE @i INT;
SET @i = 3.2 + 0.2;
You wouldn't suggest that 3.2 + 0.2 is an integer, would you? It's 3.4, a decimal. Only because of the assignment is there an implicit conversion to INT.
There's no magic in most programming languages.
SELECT SCOPE_IDENTITY()
如您所见，返回 BIGINT
SELECT @X = SCOPE_IDENTITY()
返回 BIGINT 并将其转换为 INT 变量 @X
因此，您将返回 BIGINT SCOPE_IDENTITY 并同时将其转换为 INT 并将结果设置为 @X。
返回 @X 返回 INT 结果。
SQL Server 7.0 联机丛书还声明：“建议使用 SET @local_variable 进行变量赋值，而不是 SELECT @local_variable。”