SQL查询以形成数据

Question

Mysql:

Table_A
------------
id name
1  Name1
2  Name2


Table_B
------------
a_id type  value
1    width  100
1    height 100
1    color  red
2    width  50
2    height 80

It's unknown how many type values exist in Table_B.

how to get result as:

id  name  width height  color
1  Name1  100  100      red
2  Name2  50    80      null

Answer 1

SELECT a.id, a.name, b_Width.value AS width, b_Height.value AS height, b_color.value AS color
FROM Table_A AS a
JOIN Table_B AS b_Width
   ON b_Width.a_id = a.id AND b_Width.type = 'width'
JOIN Table_B AS b_Height
   ON b_Height.a_id = a.id AND b_Height.type = 'height'
JOIN Table_B AS b_Color
   ON b_Color.a_id = a.id AND b_Color.type = 'color'

But seriously consider redesigning your schema. value is holding colors and linear dimensions, it'd be better if it were designed differently.

Keep TableA the way it is but then have a table called Details that has width/height/color columns. Or have a table called Size with width/height columns and a table called Color with the color name or RGB value. Each additional table of course has an FK to TableA which may or may not be used as that table's PK.

Answer 2

Well, I wouldn't recommend using the EAV anti-pattern for databases as it basically holds unstructured data in a structured databases, but I have had to battle with it once before, so here is something much much faster then inner joins

select a.id, a.name, max(case when b.type='height' then b.value end) as height, max(case when b.type='width' then b.value end) as width, max(case when b.type='color' then b.value end) as color from test.tablea a , test.tableb b where a.id = b.a_id group by a.id

Answer 3

use select query with sub Query

like select id,Name,(select type from table_B where a_id = table_A.id and type='width') as width from table_A

same like this you can add another columns you need ok