SQL IF ELSE / CASE clause in WHERE condition

问题

I have a very large MySQL statement looped through php foreach and each loop connected to the previous with union all. I will simplify the statement to the core of my problem, if needed I can of course also add more details later on request.

I have this table

+--------+-----------+-----------+
|   ID   |   LANG    |   TITLE   |
+--------+-----------+-----------+
|   1    |    EN     |   T-A     |
+--------+-----------+-----------+
|   1    |    FR     |   T-A     |
+--------+-----------+-----------+
|   2    |    FR     |   T-B     |
+--------+-----------+-----------+
|   3    |    DE     |   T-C     |
+--------+-----------+-----------+
|   3    |    EN     |   T-C     |
+--------+-----------+-----------+

I want to write a WHERE condition in the SQL SELECT that should show me for each ID maximum one result. But it should show results only if LANG is FR or EN. On top FR should be prefered and EN should only be displayed as alternative if no FR is available for the ID. So the result would look like this.

+--------+-----------+-----------+
|   ID   |   LANG    |   TITLE   |
+--------+-----------+-----------+
|   1    |    FR     |   T-A     |
+--------+-----------+-----------+
|   2    |    FR     |   T-B     |
+--------+-----------+-----------+
|   3    |    EN     |   T-C     |
+--------+-----------+-----------+

I have tried to build something by myself with IF - ELSE / CASE but I am not very experienced with SQL so any help would be much appreaciated.

A simplified SQL I tried would be something like

SELECT * FROM `table` 
WHERE `table`.`ID` = 1
IF `table`.`LANG` = 'FR' 
BEGIN
  AND `table`.`LANG` = 'FR' 
END
ELSE
BEGIN
  AND `table`.`LANG` = 'EN' 
END

union all 
SELECT * FROM `table` 
WHERE `table`.`ID` = 2
IF `table`.`LANG` = 'FR' 
BEGIN
  AND `table`.`LANG` = 'FR' 
END
ELSE
BEGIN
  AND `table`.`LANG` = 'EN' 
END

union all 
SELECT * FROM `table` 
WHERE `table`.`ID` = 3
IF `table`.`LANG` = 'FR' 
BEGIN
  AND `table`.`LANG` = 'FR' 
END
ELSE
BEGIN
  AND `table`.`LANG` = 'EN' 
END

Sitenote I may not use any construct with ORDER BY combined with LIMIT 1 since I am looping the SQL through a php for each loop multiple times.

EDIT: SOLUTION that worked for me

SELECT * FROM `table1`
WHERE ID = 1
AND lang = 'FR'
OR (lang = 'EN' AND ID NOT IN (SELECT ID FROM table1 WHERE lang = 'FR'))

回答1:

The most elegant and efficient solution with a sophisticated query optimizer would be this:

SELECT * FROM (
    SELECT * FROM `table`
    WHERE ID IN (
      SELECT id FROM `table` 
      WHERE lang = 'EN'
      EXCEPT
      SELECT id FROM `table`
      WHERE lang = 'FR'
    ) OR table.LANG ='FR'
) t1
WHERE id = ?

This gives you the desired result, filtered by ID. In case the optimizer is however not able to push down the id = ? you might have to do it yourself to get decent performance:

  SELECT * FROM `table`
    WHERE id = ? AND (ID IN (
      SELECT id FROM `table` 
      WHERE lang = 'EN' AND ID = ?
      EXCEPT
      SELECT id FROM `table`
      WHERE lang = 'FR' AND ID = ?
    ) OR table.LANG ='FR')

However, if you can, I would get all the results at once and not iterate over the IDs in the first place:

SELECT * FROM `table`
WHERE ID IN (
  SELECT id FROM `table` 
  WHERE lang = 'EN'
  EXCEPT
  SELECT id FROM `table`
  WHERE lang = 'FR'
) OR table.LANG ='FR'

This will get you all the IDs that have lang 'EN' and no corresponding 'FR' plus all the 'FR's. Alternatively you could also try:

SELECT * FROM `table`
WHERE lang = 'FR'
OR (lang = 'EN' AND ID NOT IN (SELECT ID FROM table WHERE lang = 'FR'))

SELECT * FROM `table`
WHERE lang = 'FR'
OR (table.LANG = 'EN' AND NOT EXISTS (SELECT * FROM table t1 WHERE lang = 'FR' AND t1.id = table.id))

But my guess would be the first query is fastest.

回答2:

select * from table where lang = 'FR' union
select t_en.* from table t_en left join table t_fr 
on (t_fr.id = t_en.id and t_fr.lang = 'FR' and t_en.lang = 'EN')
where t_fr.id is null and t_en.lang = 'EN'

回答3:

You can get your exact result in your question like this:

SELECT * FROM `table`
WHERE lang = 'FR'
OR (lang = 'EN' AND ID NOT IN (SELECT ID FROM `table` WHERE lang = 'FR'))

See Fiddle-example

If you really want to give the ID each time you can do

SELECT * FROM `table`
WHERE (ID=1) AND
( lang = 'FR'
  OR (lang = 'EN' AND ID NOT IN (SELECT ID FROM `table` WHERE lang = 'FR'))
)

This gives just one result per ID (Fiddle-example).

You can also use multiple ID's

SELECT * FROM `table`
WHERE (ID in (1,2,3)) AND
( lang = 'FR'
  OR (lang = 'EN' AND ID NOT IN (SELECT ID FROM `table` WHERE lang = 'FR'))
)

and build the (1,2,3) from your script and execute the select. See Fiddle-example.

回答4:

for better knowledge of case statement in mysql

http://dev.mysql.com/doc/refman/5.0/en/case.html

for better knowledge of if statement in mysql

http://dev.mysql.com/doc/refman/5.0/en/if.html

回答5:

SELECT * FROM TABLE WHERE ( LANG='FR' OR (lang='EN' AND ID NOT IN (SELECT ID FROM TABLE WHERE lang='FR' )))

来源：https://stackoverflow.com/questions/18658785/sql-if-else-case-clause-in-where-condition

标签

php

sql

if-statement

switch-statement

case