An "EXISTS" statement is generally used to select/update/delete some rows of a table based on the content of columns of other tables.
If the "SELECT" statement used as argument of the "EXISTS" statement is always returning "true" for all rows of the main SELECT
statement, the EXISTS statement is useless and has the same effect as if it was not there. Still, this is probably not the original
intend of the developer to have an EXISTS statement that is always true.
As a consequence, the SELECT statement of an EXISTS statement should always contain a WHERE clause.
What is true for EXISTS is also true for NOT EXISTS.
Noncompliant code example
SELECT *
FROM sys.[databases] AS [sd]
WHERE EXISTS (SELECT 1
FROM [sys].[master_files] AS [mf])
Compliant solution
SELECT *
FROM sys.[databases] AS [sd]
WHERE EXISTS (SELECT 1
FROM [sys].[master_files] AS [mf]
WHERE [mf].[database_id] = [sd].[database_id])
Exceptions
This rule doesn’t raise an issue when EXISTS is used in the context of a WHILE or a IF statement.
