情况下，MySQL在哪儿查询

-2

表

lan_dish_id。| lan_dish_lang | lan_dish_dishid | lan_dish_mainlang
--------------- | ------------------- | --------------------- | ------------------------
| NL | 8 | 1
| FR | 8 | 0

我需要检查$_SESSION['lang']中的语言是否在表中可用，否则它必须采用主语言。

不幸的是，当运行下面的查询时，他返回带有FR的行以及NL als lan_dish_lan ...在这种情况下，它只能返回带有FR的行。

SELECT * FROM `language_dish` WHERE 
(CASE 
WHEN lan_dish_lang = "fr" 
THEN lan_dish_lang = "fr" 
ELSE lan_dish_mainlang = 1 
END) AND lan_dish_dishid = 8

来源

2017-10-01 Tim Biesmans

你问[同样的问题，用不同的标题（https://stackoverflow.com/questions/46511749/using-case-in-where-clause） 3小时前 – Mawg

当case语句在where子句中满足时，Sql不会停止读取行，并且您的where子句对两个行的计算结果都为true，则返回两行。 –

这里有两种方法可能适合你。

/* 
DROP TABLE IF EXISTS T; 

CREATE TABLE T (lan_dish_id INT, lan_dish_lang VARCHAR(2), lan_dish_dishid INT, lan_dish_mainlang INT); 
*/ 
truncate table t; 
INSERT INTO T VALUES 
(1     , 'nl' , 8,       1) , 
(4     , 'fr' , 8,       0) , 
(5     , 'nl' , 9,       1) , 
(6     , 'de' , 7,       0) 

; 

select s.lan_dish_dishid, left(gc,instr(concat(gc,','),',') -1) lang 
from 
(
select lan_dish_dishid, group_concat(lan_dish_lang order by lan_dish_mainlang) gc 
from t 
#where lan_dish_dishid = 8 
group by lan_dish_dishid 
order by lan_dish_dishid 
) s 
; 
select s.lan_dish_id, s.lan_dish_lang 
from 
(
select t.*, 
     if(t.lan_dish_dishid <> @p , @rn:=1,@rn:[email protected]+1) rn, 
     @p:= t.lan_dish_dishid p 
from t , (select @rn:=0,@p:=0) r 
order by t.lan_dish_dishid, t.lan_dish_Mainlang 
) s where s.rn = 1

结果

+-------------+---------------+ 
| lan_dish_id | lan_dish_lang | 
+-------------+---------------+ 
|   6 | de   | 
|   4 | fr   | 
|   5 | nl   | 
+-------------+---------------+ 
3 rows in set (0.01 sec)

来源

2017-10-01 11:05:26

我明白了，但这不是必需的结果。如果存在lan_dish_lang列中具有FR的行，则需要显示该行。否则，如果它不存在，它需要显示那个lan_dish_mainlang = 1的那个特定lan_dish_dishid行。我希望你能理解它... –

我都是agog--给出我发布的样本数据的预期结果是什么？ –

我收到FR的那一行。好吧，但如果FR行不存在，它需要显示列lan_dish_mainlang = 1 –

情况下，MySQL在哪儿查询

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