SQL函数avg（）始终返回true（空）

Question

Whenever i use the AVG() function in mysql, it will always return a value or null. Which means, even is there is no result, it will still returns the rows as null.

Here is the sql command I'm using:

SELECT teams.id, AVG(players.skill) AS "rating", teams.name, credit, date, leagues.name AS "league_name"
FROM (teams)
LEFT JOIN leagues ON leagues.id = teams.league
LEFT JOIN players ON teams.id = players.team
WHERE manager = '20'
LIMIT 1

As you can see, i select where manager id equal 20, this manager doesn't exist, and it still returns rows as null.

I can see alot of that this issue, however I cant seem to find a thread that helps me.

Answer 1

Try this way:

SELECT teams.id,  
       teams.name, 
       credit, 
       date, 
       leagues.name AS "league_name",
       AVG(players.skill) AS "rating"
  FROM teams
     LEFT JOIN leagues ON leagues.id = teams.league
     LEFT JOIN players ON teams.id = players.team
 WHERE teams.manager = '20'
 GROUP BY teams.id,  
       teams.name, 
       credit, 
       date, 
       leagues.name
 LIMIT 1

The GROUP BY statement is used in conjunction with the aggregate functions (max, min, avg, etc) to group the result-set by one or more columns.