Question

I understand that WHERE 1 = '1' returns true, and also WHERE 222 = CONVERT(varchar, 222) will return true in T-SQL.

But, when working with IN, can the test_expression and the subquery be of different datatypes? For example can the test_expression be an int, and the subquery varchar?

Answer 1

They can be: all datatypes will be cast to the highest based on “datatype precedence” rules.

Logically, x in (a,b,c) is actually x=a or x=b or x=c of course. This is relevant for this contrived case using scalar values. I really don’t know if all values are CAST to float (the highest here) before comparison (..IN..) or cast per comparison (..OR..OR..)

WHERE
  CAST(1 AS int) IN ('1', CAST(1.0 AS float), 1.0 /*decimal*/)

For a subquery in the IN clause, then the column has a data type that will be CAST as per the scalar rules

Edit, after comment:

All datatypes will be implicitly CAST

SELECT TOP 1 *
FROM sys.columns
WHERE
    1 IN (SELECT CAST('1' AS CHAR(1)) FROM sys.columns)

or

DECLARE @intvar int = 1, @charvar char(1) = '1';

SELECT TOP 1 *
FROM sys.columns
WHERE
    @intvar IN (SELECT @charvar FROM sys.columns)