SQL 计算表 2 中表 1 中特定值出现的次数

Question

我有 2 张桌子，table1 所选食物的名称 table2 为用户选择的早餐、午餐、晚餐、...

表 1：

id name
0 Apple
1 watermelon
2 Pear
3 Peach
4 Walnut
. .
200 Apricot

表 2：

id  username    breakfast  lunch  dinner  Meal1   Meal2  Meal3  
0   Dave        apple      null   Pear    null    null   watermelon
1   John        walnut     Peach  Pear    Apricot null   apple
.   ....        ......     .....  ....    ....... ....   .....
900 Ray         walnut     Peach  Pear    Apricot null   apple

注意：用户在用餐时不能多次选择苹果。

我想知道每位厨师为这位客人准备的食物数量：

apple 400
Pear  399
Peach 398

我试过了：

SELECT breakfast, COUNT( * ) AS value
FROM table2
WHERE breakfast !=  ""
GROUP BY breakfast
ORDER BY `value` DESC
LIMIT 0 , 30

但这不是我想要的，如果用户只想要晚餐，结果中不会出现食物的名称。 谢谢。

Answer 1

感谢Strawberry 它可以通过规范化 table2 来完成

SELECT name, value
FROM (    
SELECT breakfast AS name, COUNT( * ) AS value
FROM table2
WHERE breakfast !=  ""
GROUP BY breakfast
UNION 
SELECT lunch AS name, COUNT( * ) AS value
FROM table2
WHERE lunch !=  ""
GROUP BY lunch 
UNION 
SELECT dinner AS name, COUNT( * ) AS value
FROM table2
WHERE dinner !=  ""
GROUP BY dinner
UNION 
SELECT meal1 AS name, COUNT( * ) AS value
FROM table2
WHERE meal1 !=  ""
GROUP BY meal1
UNION 
SELECT candid5 AS name, COUNT( * ) AS value
FROM table2
WHERE meal2 !=  ""
GROUP BY meal2
UNION 
SELECT meal3 AS name, COUNT( * ) AS value
FROM table2
WHERE meal3 !=  ""
GROUP BY meal3
UNION 
SELECT meal4 AS name, COUNT( * ) AS value
FROM table2
WHERE meal4 !=  ""
GROUP BY meal4
) AS mynewtablename
GROUP BY name
ORDER BY  `mynewtablename`.`value` DESC

希望其他人向Strawberry 学习

它可以帮助某人。