Question

I have 3 tables :

tb_a                tb_b             tb_c
==============      ==============   ==============
|id|doc1|urla|      |id|doc2|urlb|   |id|doc3| sum|
==============      ==============   ==============
| 1| c1 |url1|      | 1| c2 |urla|   | 1| c2 |sum1|
| 2| c3 |url3|      | 2| c4 |urlb|   | 2| c3 |sum3|
| 3| c2 |url2|      | 3| c6 |urlc|   | 3| c4 |sum4|
==============      ==============   | 4| c6 |sum6|
                                     ==============

First, I wanna check tb_a and tb_b. If there’re same data in doc1 and doc2, I just use data doc1 in tb_a. here’s first query :

SELECT 
   a.doc1
FROM 
   tb_a a
LEFT JOIN 
   tb_b b
   ON a.doc1 = b.doc2
WHERE 
   b.doc2 IS NULL 

then left join with tb_c to get sum. can I make those processes in one query? how? thank you 🙂

from the example, so the result must be :

c3 url3 sum3 
c2 url2 sum1
c4 urlb sum4
c6 urlc sum6

Answer 1

You can join all the tables using LEFT JOIN with COALESCE.

SELECT  a.doc3 DOCValue, 
        COALESCE(b.urla, c.urlb) AS URLValue,
        a.`SUm` AS SUMValue
FROM    tb_c a
        LEFT JOIN tb_a b
            ON a.doc3 = b.doc1
        LEFT JOIN tb_b c
            ON a.doc3 = c.doc2

SQLFiddle Demo