在一对多关系中，我想找到满足许多特定条件的数据

Question

Product Table

id(pk)	name
1	example name1
2	example name2

Attribute Table

id(pk)	product_id(fk)	attr
1	1	new
2	1	blue
3	1	car
4	2	new

In the above table, i want to find a specific product_id with both 'new' and 'blue' attributes(In this situation, expected value is product_id 1)

select product_id form attrbute_table where attr in('new', 'blue') 

did not give me the answer I was looking for

Answer 1

I can propose two solutions:

• We can run 2 sub queries, one for each attribute, and then return only the productID's which are in both result sets.
• We can count the number of line returned which contain "blue" or "new".
  NB: If 2 lines are returned we assume that one is "blue" and one is "new". If your data could return 2 lines with the same PID and both "blue" this query would return a false positive. We could around this with a sub-query with DISTINCT but it would not be any simpler than solution 1.

 create table attribute ( ID int primary key, PID int, attr VARCHAR(10));

 ✓ 

 insert into attribute values (1,1,'new'), (2,1,'blue'), (3,1,'car'), (4,2,'new'), (5,3,'blue');

 ✓ 

 SELECT a.PID FROM (SELECT PID, attr FROM attribute WHERE attr ='blue') a JOIN (SELECT PID,attr FROM attribute WHERE attr ='new') b ON a.PID = b.PID;

 |  PID |  |  --: |  |  1 | 

 SELECT PID product_id, COUNT(attr) num_found FROM attribute WHERE attr = 'blue' or attr = 'new' GROUP BY PID HAVING COUNT(attr) = 2;

 product_id | num_found ---------: |  --------: 1 |  2 

db<>fiddle here

Answer 2

'i want to find a specific product_id with both 'new' and 'blue' attributes'

so you can use and condition

select product_id form attrbute_table where attr = 'new' and attr = 'blue' 

The IN operator allows you to specify multiple values in a WHERE clause.

The IN operator is a shorthand for multiple OR conditions . https://www.w3schools.com/sql/sql_in.asp