IF语句中的MySQL SELECT WHERE

Question

I have a "group_names" table which stores user groups. Each group has a manager, which can be either an individual user, or be managed by another group. This is currently determined by group_names.mgr_type. An INT value of 1 means that it's an individual user and a value of 2 means that it's a group that manages it.

For example, the "General Users" group could be managed by the "IT Department" group or by "John Doe". Unique ID fields on the users and group_names tables are "user_id" and "group_id".

I want to take the logic out of PHP and just put it in the SQL query, but I'm running into problems here. I'm not sure if this is possible given the table layout or if I'm just incapable of following simple instructions provided in the documentation.

SELECT gn.group_id, gn.group_name, ... gn.mgr_id, CONCAT(usr.user_firstname,' ',usr.user_lastname) AS created_name, usr.user_id,
CASE gn.mgr_type
    WHEN '1' THEN CONCAT(usr.user_firstname,' ',usr.user_lastname) WHERE usr.user_id=gn.mgr_id LIMIT 1
    WHEN '2' THEN gn.group_name WHERE gn.group_id=gn.mgr_id LIMIT 1
END AS mgr_name
FROM group_names gn 
LEFT JOIN users usr 
ON gn.created_by=usr.user_id 
ORDER BY group_name ASC;

I've tried different variations of IF and CASE, with and without parentheses, but keep getting the "invalid syntax" error.

...for the right syntax to use near 'WHERE usr.user_id=gn.mgr_id LIMIT 1) WHEN '2' THEN (gn.group_name WHERE gn' at line 3

Can I even select a different row from the same table (select the group_name from a different row based on the mgr_id=group_id), or should I add another table? Any suggestions on an easier/more efficient way to do this?

Thanks!

Answer 1

I think you should try something like this(try shifting where in the join conditions of another instance of table):

 SELECT gn.group_id, gn.group_name, ... gn.mgr_id,
        CONCAT(usr.user_firstname,' ',usr.user_lastname) AS created_name, 
        usr.user_id,
       CASE gn.mgr_type
         WHEN '1' THEN CONCAT(usr1.user_firstname,' ',usr1.user_lastname)
         WHEN '2' THEN gn1.group_name
      END AS mgr_name
 FROM group_names gn 
 LEFT JOIN users usr 
 ON gn.created_by=usr.user_id
 RIGHT OUTER JOIN users usr1
 ON usr1.user_id=gn.mgr_id LIMIT 1
 OUTER JOIN group_names gn1
 ON usr.user_id=gn1.mgr_id LIMIT 1
 ORDER BY group_name ASC;

Answer 2

Have you tried this since you already joined to those tables

CASE gn.mgr_type
    WHEN '1' THEN CONCAT(mgr_usr .user_firstname,' ',mgr_usr .user_lastname)
    WHEN '2' THEN mgr_grp.group_name
END AS mgr_name

You actually need 2 more left joins for that to work

LEFT JOIN users mgr_usr 
   ON gn.mgr_id = mgr_usr.user_id 
LEFT JOIN group_names mgr_grp
   ON gn.mgr_id = mgr_grp.group_id

Left join to the user table to get the manager name if it's user or join to group_names table to retrieve the group name if it's a group

You can restrict the JOINs further

LEFT JOIN users mgr_usr 
   ON gn.mgr_id = mgr_usr.user_id AND gn.mgr_type = 1
LEFT JOIN group_names mgr_grp
   ON gn.mgr_id = mgr_grp.group_id AND gn.mgr_type = 2

Answer 3

try this:

SELECT ...
CASE WHEN gn.mgr_type = '1' 
    THEN CONCAT(usr.user_firstname,' ',usr.user_lastname) WHERE usr.user_id=gn.mgr_id LIMIT 1
    ELSE gn.group_name WHERE gn.group_id=gn.mgr_id LIMIT 1
    END AS mgr_name
FROM group_names gn...