用条件连接 3 个表

Question

Need to get the results from 3 tables.

Table1: users

Attr.:user_id, user_code, active

Table2: user_addresses

Attr.: user_address_id, user_id, [address fields], active

Table3: user_details

Attr.: user_detail_id, user_id, [detail fields], active

active is the boolean field. In user_addresses and user_details , an user_id can have more than one row (say 3 rows). 2 rows will be having active field set to 0 (inactive). The 3rd row is the active one with active field set to 1.

Now, I want to join all these tables when I search with user_code . I compiled the following join query:

SELECT "*" FROM "users" 
LEFT JOIN "user_details" AS "ud" USING ("user_id") 
LEFT JOIN "user_addresses" AS "ua" USING ("using_id") 
WHERE "user_code" = 'TRY001' AND "users"."active" = 1 AND "ua"."active" = 1 AND "ud"."active" = 1

I get the result as expected only when there are entry in all tables. So I changed to something like this:

SELECT "*" FROM "users" 
LEFT JOIN "user_details" AS "ud" USING ("user_id") 
LEFT JOIN "user_addresses" AS "ua" USING ("using_id") 
WHERE "user_code" = 'TRY001' AND "users"."active" = 1 OR "ua"."active" = 1 OR "ud"."active" = 1

This time I get the first row (which is inactive), rather than active one.

What I need actually is:

1. Join three tables
2. Get only the active rows from user_addresses and user_details tables
3. Incase there are no active rows / no record exist in user_addresses and user_details table, I just want to return the column name with NULL values in it.

Answer 1

When you use left join , the conditions on all but the first table need to go into the ON clauses:

SELECT *
FROM users u LEFT JOIN
     user_details ud
     ON ud.user_id = u.user_id AND ud.active = 1 LEFT JOIN 
     user_addresses ua 
     ON ua.using_id = ud.using_id AND ua.active = 1
WHERE u.user_code = 'TRY001' AND u.active = 1 ;

I am guessing where the columns are coming from, particularly using_id .

The reason these need to go into the ON clauses is because of what happens when there is no match. The value is NULL and the WHERE conditions fail.