MySql仅从父键具有多个值的子表中获取记录

Question

I have 2 tables parent and child. Parent has a field called unique_id and child has a field called parent_unique_id and it is the foreign key in the child table. Parent table has 4 records for the same unique_id and child table has 5 records for the same unique_id.

When I join them to get the records, I am getting a total of 20 records each 5 records of the child table are repeating 4 times.

The query I am using is

SELECT c.* FROM child c JOIN parent p ON c.parent_unique_id = p.unique_id

I tried LEFT JOIN as well but I am still getting 20 records which are repeated.

Answer 1

One method is to use IN or EXISTS :

SELECT c.*
FROM child c 
WHERE c.parent_unique_id IN (SELECT p.unique_id FROM parent p);

Of course, you could just slap SELECT DISTINCT on your query. However, that requires and unnecessary processing.