How do I use an IF statement in an MySQL join query?

Question

I have a SQL query that left joins a table in different ways depending on a condition.

SELECT m.id, u.first_name AS otherUser
FROM matches AS m
IF (u.id=m.user2I         


        

Answer 1

Thanks, It worked for me. Tried something else like below :

**

Create table TEMP1 (ID INT,T_MONTH INT,T_YEAR INT)
INSERT INTO TEMP1 VALUES(1,1,2001)
INSERT INTO TEMP1 VALUES(2,2,2001)
INSERT INTO TEMP1 VALUES(3,3,2001)

**

CREATE TABLE TEMP2 (T_MONTH INT,T_YEAR INT,FREQUENCY CHAR(1),VAL FLOAT)
INSERT INTO TEMP2 VALUES(1,2001,'M',1.1)
INSERT INTO TEMP2 VALUES(3,2001,'M',1.2)
INSERT INTO TEMP2 VALUES(3,2001,'Q',1.3)
INSERT INTO TEMP2 VALUES(12,2001,'A',1.4)

SELECT * FROM TEMP1 L JOIN TEMP2 H
ON L.T_YEAR = H.T_YEAR
OR (L.T_MONTH = (CASE WHEN H.FREQUENCY='M'
                       THEN H.T_MONTH
                  END)
    OR dbo.GetQuarterFromMonth(L.T_MONTH) = (CASE WHEN H.FREQUENCY = 'Q'
                        THEN dbo.GetQuarterFromMonth(H.T_MONTH)
                        END))
WHERE H.FREQUENCY = 'Q'

Here GetQuarterFromMonth is a user defined funtion which returns Quarter for a particular month.

The objective of above query is to inflate the value of Quarter for all the months in table 1. If frequency is annual then in that case the value should be inflated for all the months 1-12 of table 1 which is achieved through first join condition. In case frequency is Monthly then each month from table1 should be mapped to table2. The only unique case is handling quarter frequency.