MySQL，WHERE 子句中的 IF 语句？

Question

Table:

odit_docs - table

id|name |regNumber|mps_id
1 |test1|123      | 1
2 |test2|124      | NULL

mps - table

id|name |mpsRegnomer|
1 |test1|1233       |
2 |test2|1244       |

The query:

SELECT * FROM `odit_docs` as od where IF(od.mps_id IS NOT NULL, SELECT mps.mpsRegnomer FROM mps where mps.id = od.mps_id, od.regNumber) = '123'

The scenario:

If column od.mps_id IS NOT NULL to select value from other table mps based on od.mps_id <=> mps.id relationship, but if column od.mps_id is NULL it should take column od.regNumber at the end I should check if returned value from this if-else statement is equal to some value in this case it's 123 I got error in this form of query.

In case with id 1 it should check if value from mps table mps.mpsRegnomer = 123 , because od.mps_id IS NOT NULL

In case with id 2 it should check if od.regNumber = 123 , because od.mps_id IS NULL

Answer 1

This is your query with fixed syntax:

SELECT * 
FROM odit_docs as od 
where IF(od.mps_id IS NOT NULL, (SELECT mps.mpsRegnomer FROM mps where mps.id = od.mps_id LIMIT 1), od.regNumber) = '123';

This will do the same thing but I believe it will generate a better execution plan:

SELECT * 
FROM odit_docs as od 
LEFT JOIN mps ON mps.id = od.mps_id
WHERE (od.mps_id IS NULL and od.regNumber = '123')
      OR (mps.id = '123')

Here's a sqlfiddle where you can test it: http://sqlfiddle.com/#!9/61e101/4