将行值显示为 SQL 中的列值

Question

我在 SQL(XAMP)

中有这个 table

item_name         category_name        category_id
----------------------------------------------------
Flash_disk        Accessories          1    
Fanta             Food                 2    
Printer           Accessories          1    
Sprite             Food                2

我正在尝试使用此查询将 category_name 列值显示为列值，它显示得非常好，如图所示。 sql:

SELECT category_id, MAX(IF(`category_name` = "Food", item_name, NULL)) Food, MAX(IF(`category_name` = "Accessories", item_name, NULL)) Accessories
FROM categories
GROUP BY category_id

结果是

category_id         Food           Accessories
---------------------------------------------------
1                   NULL            Printer
2                   Sprite           NULL
---------------------------------------------------

有没有一种方法可以让所有列都填充值而不是显示空值结果是这样的：

category_id         Food           Accessories
---------------------------------------------------
1                   Fanta          Printer
2                   Sprite         Flash_disk   
---------------------------------------------------

Answer 1

你可以使用 coalesce():

SELECT category_id,
       COALESCE(MAX(CASE WHEN `category_name` = 'Food' THEN  item_name END), 'Fanta') as Food, 
       COALESCE(MAX(CASE WHEN `category_name` = 'Accessories' THEN  item_name END), 'Flash_disk') as Accessories
FROM categories
GROUP BY category_id;

Answer 2

预期的结果没有多大意义。

例如，根据这些结果，芬达为什么会有 category_id 1？

但是这个查询几乎得到相似的结果：

SELECT rownum, 
MAX(CASE WHEN category_name = 'Food' THEN item_name END) AS Food, 
MAX(CASE WHEN category_name = 'Accessories' THEN item_name END) AS Accessories
FROM
(
    SELECT 
    MAX(cat1.category_name) AS category_name,
    COUNT(*) AS rownum,
    cat1.item_name
    FROM categories cat1
    JOIN categories cat2 ON cat2.category_id = cat1.category_id 
     AND cat2.item_name <= cat1.item_name
    GROUP BY cat1.category_id, cat1.item_name
) q
GROUP BY rownum
ORDER BY rownum;

Returns

rownum  Food    Accessories
1       Fanta   Flash_disk
2       Sprite  Printer

将行值显示为 SQL 中的列值

Displaying row values as column values in SQL

mysql

sql

wamp