为什么我的查询没有返回任何结果？

Question

Why does this sql query only show results if they only have a row in users_warnings?

SELECT 
 u.id, 
 uw.warning 
FROM 
 users u
 INNER JOIN users_warnings uw ON (
  u.id = uw.uID
 )
LIMIT 21

I wish to show all users, but also grab the column "warning" in users_warnings, if any.

Answer 1

Because you're using an inner join , which requires a row to exist on the joined table. Try the following:

SELECT 
 u.id, 
 uw.warning 
FROM 
 users u
 LEFT JOIN users_warnings uw ON (
  u.id = uw.uID
 )
LIMIT 21

Answer 2

An INNER JOIN only returns something if there is data in both tables.
Try a LEFT JOIN instead:

SELECT u.id, uw.warning FROM users u
            LEFT JOIN users_warnings uw ON (u.id = uw.uID)
            LIMIT 21

Answer 3

Change your inner join for a left join, as so:

SELECT u.id, uw.warning FROM users u
                LEFT JOIN users_warnings uw ON (u.id = uw.uID)
                LIMIT 21

Answer 4

SELECT  
    u.id,  
    IFNULL(uw.warning,'') warning
FROM  
   (SELECT id FROM users LIMIT 21) u 
   LEFT JOIN users_warnings uw
   ON u.id = uw.uID 
;

• Should be a LEFT JOIN not INNER JOIN
• Refactored the query to get first 21 users before attempting a JOIN (faster query)
• Defaulted warning to blank string if user had no warning