Question

I have a prepared query in two versions, the first one works but the second using CASE WHEN doesn’t :

 SELECT  mFrom.user_name AS sender_name, mp.message AS message
 FROM msg_pv mp
 INNER JOIN membresmvc AS mFrom 
    ON mp.id_from = mFrom.id
 WHERE mp.id_to = :id_member

CASE WHEN version :

 SELECT  mFrom.user_name AS sender_name, mp.message AS message
 FROM msg_pv mp
 INNER JOIN 
    CASE mp.from_type
        WHEN "mb" THEN membresmvc AS mFrom ON mFrom.id = mp.id_from 
    END -- also tried "END CASE"    
 WHERE mp.id_to = :id_member

The value of mp.from_type is always “mb” in the msg_pv table. I exactly use the structure recommented in the MySQL reference manual. I don’t understand why the second query doesn’t work.

Answer 1

If your CASE espression value always evaluates to true, there is no need to have that statement.

However, I think the usage of the CASE in your query is wrong. It should be something like as shown below. You can have the CASE on the ON condition but not on the JOIN itself. This way you can join to different columns based on a condition.

This is syntactically correct but not sure what the output would be.

You can validate this statement using this link: Instant SQL Formatter

Query:

SELECT      mFrom.user_name AS sender_name, 
            mp.message      AS message 
FROM        msg_pv mp 
INNER JOIN  membresmvc 
ON          ( 
                CASE mp.from_type 
                    WHEN "mb" THEN mFrom.id = mp.id_from 
                    ELSE NULL 
                END 
            ) 
WHERE  mp.id_to = :id_member;

UPDATE

If you need to join to multiple tables based on a condition, then you need to make use of LEFT OUTER JOIN. You query might look something like this assuming your second table is *second_table* and another value is “another value”

SELECT          COALESCE(mFrom.user_name, second_table.some_col) AS sender_name, 
                mp.message      AS message 
FROM            msg_pv mp 
LEFT OUTER JOIN membresmvc 
ON              mFrom.id        = mp.id_from 
AND             mp.from_type    = "mb"
LEFT OUTER JOIN second_table 
ON              second_table.id = mp.id_from 
AND             mp.from_type    = "another value"
WHERE           mp.id_to = :id_member;

EXAMPLE

Here is an example that illustrates it. This script was tested in SQL Server 2012 database. It might differ in MySQL. However, the concept of LEFT OUTER JOIN used here is same even in MySQL.

• There are three tables namely table1, table2 and table3
• Here we have to join table1 with both table2 and table3 based on the below conditions.
• Join colrefid in table1 with col2 in table2 if constant in table1 has the value t2.
• Join colrefid in table1 with col3 in table3 if constant in table1 has the value t3.
• You can see the output at the end

Hope that gives you an idea to join the tables as per your requirements.

Script:

CREATE TABLE dbo.Table1
(
        col1        INT         NOT NULL
    ,   colrefid    INT         NOT NULL
    ,   constant    VARCHAR(10) NOT NULL
);

CREATE TABLE dbo.Table2
(
        col2    INT         NOT NULL
    ,   name    VARCHAR(10) NOT NULL
);

CREATE TABLE dbo.Table3
(
        col3    INT         NOT NULL
    ,   name    VARCHAR(10) NOT NULL
);

INSERT INTO dbo.Table1 (col1, colrefid, constant) VALUES
    (123, 2, 't2'),
    (784, 3, 't3'),
    (498, 2, 't2');

INSERT INTO dbo.Table2 (col2, name) VALUES
    (2, 'table 2');

INSERT INTO dbo.Table3 (col3, name) VALUES
    (3, 'table 3');

SELECT          t1.col1
            ,   t1.colrefid
            ,   t1.constant
            ,   COALESCE(t2.col2, t3.col3) colvalue
            ,   COALESCE(t2.name, t3.name) colname
FROM            dbo.Table1  t1
LEFT OUTER JOIN dbo.Table2  t2
ON              t2.col2     = t1.colrefid
AND             t1.constant = 't2'
LEFT OUTER JOIN dbo.Table3  t3
ON              t3.col3     = t1.colrefid
AND             t1.constant = 't3';

Output:

col1 colrefid constant colvalue colname
---- -------- -------- -------- -------
123      2       t2        2    table 2
784      3       t3        3    table 3
498      2       t2        2    table 2