SQL Server存在返回类型

Question

What does the SQL Server Exists function return as a type?

Does Exists have a conceptual return type?

SELECT * 
FROM tableName 
WHERE EXISTS (SELECT * 
              FROM tableName 
              WHERE columnName LIKE 'theValue%') = 1

Why does this not work?

Answer 1

As documented in BOL

Result Types

Boolean

It has a return type of boolean but this is just an internal datatype currently you can't declare columns or variables of that datatype.

Or use it in comparisons - even against another boolean.

where exists(select 1) = exists(select 1)

Also fails.

SQL Server doesn't implement the SQL Standard boolean datatype yet - the closest equivalent is bit but this is not a true boolean.

Answer 2

Exists specifies a subquery to test for the existence of rows and returns boolean(True if any row exists else false). Microsoft docs So your code should be

SELECT * 
FROM tableName 
WHERE EXISTS (SELECT * 
          FROM tableName 
          WHERE columnName LIKE 'theValue%')

This will return all rows from table 'tableName' IF any one of the rows has 'columnName ' as 'theValue%' ELSE none of the rows will be returned.