Not a GROUP BY expression error [duplicate]

Answer 1

SELECT MEMBERS.MEMBER_ID,
       MEMBERS.MEMBER_NAME, 
       COUNT(personal_training_sessions.session_id)

FROM MEMBERS JOIN personal_training_sessions
ON personal_training_sessions.member_id=members.member_id

GROUP BY personal_training_sessions.session_id;

You are using a COUNT function, thus the other columns, MEMBER_ID & MEMBER_NAME, must be included in the group by clause.

Answer 2

The error says it all, you're not grouping by MEMBERS.MEMBER_ID and MEMBERS.MEMBER_NAME.

SELECT MEMBERS.MEMBER_ID, MEMBERS.MEMBER_NAME
     , COUNT(personal_training_sessions.session_id)
  FROM MEMBERS 
  JOIN personal_training_sessions
    ON personal_training_sessions.member_id = members.member_id
 GROUP BY MEMBERS.MEMBER_ID, MEMBERS.MEMBER_NAME

You want the count of personal sessions per member, so you need to group by the member information.

The basic (of course it can get a lot more complex) GROUP BY, SELECT query is:

SELECT <column 1>, <column n>
     , <aggregate function 1>, <aggregate function n>
  FROM <table_name>
 GROUP BY <column 1>, <column n>

An aggregate function being, as Ken White says, something like MIN(), MAX(), COUNT() etc. You GROUP BY all the columns that are not aggregated.

This will only work as intended if your MEMBERS table is unique on MEMBER_ID, but based on your query I suspect it is. To clarify what I mean, if your table is not unique on MEMBER_ID then you're not counting the number of sessions per MEMBER_ID but the number of sessions per MEMBER_ID and per MEMBER_NAME. If they're in a 1:1 relationship then it's effectively the same thing but if you can have multiple MEMBER_NAMEs per MEMBER_ID then it's not.