SQL JOIN同一表上的两个不同列

Question

I have a MySQL Database Table that indicates the relations between some entries in my database. the table has 4 columns: (the table name is: RelationsTable)

 id |  type | relationId | relationType
 1 |  customer | 5 | recipt
 2 |  recipt | 4 | customer

I'm trying to add this table's data to another table that contains a customer ID. But since the customer's ID might appear both as RelationsTable.id or RelationsTable.relationId , I can't use a normal JOIN. What should I do?

thanks!

Answer 1

you just need to link it, try this

select * from table1, table2 where table1.customer_id = table2.customer_id;

if still error change to

select * from table1, table2 where table1.customer_id == table2.customer_id;

Answer 2

您必须两次将客户表连接到两个具有不同客户表别名的列。

Answer 3

I'm just guessing here with not much info from your post.

You probably need this:

SELECT RT.id, Cust.Lastname, Cust.Firstname
FROM RelationsTable RT INNER JOIN Customers Cust
ON RT.id = Cust.id
WHERE RT.type = 'customer'
UNION ALL
SELECT RT.relationid, Cust.Lastname, Cust.Firstname
FROM RelationsTable RT INNER JOIN Customers Cust
ON RT.relationid = Cust.id
WHERE RT.relationtype = 'customer'