Cannot use an aggregate or a subquery in an expression used for the group by list of a GROUP BY clause

Question

In the below sql statement i get the following error

Cannot use an aggregate or a subquery in an expression used for the group by list of a GROUP B

Answer 1

Unless you include T.Client in your GROUP BY, you can only include that field within an aggregate function. In your case, grouping by that field changes the logic, so that's out (and is related to your attempt to group by the CASE statement). Instead, wrap T.Client in an aggregate function.

This way your groups are still the same, and when there is only one row, as per your CASE statement's test, you know what result the aggregate funciton is going to give.

SELECT
  T.Post,
  ClientCount = COUNT(*) AS ClientCount,
  Client      = CASE COUNT(*) WHEN 1 THEN MAX(T.Client) ELSE '[Clients]' END
FROM
  MyTable T
GROUP BY
  T.Post

Answer 2

You do not need to group by that CASE expression.

SELECT
    T.Post,
    COUNT(*) AS ClientCount,
    CASE COUNT(*) WHEN '1' THEN MIN(T.Client) ELSE '[Clients]' END Client
FROM
    MyTable T
GROUP BY
    T.Post