Question

I have two existing tables with data populated

table A
–tableA_id
–contentA

table B
–tableB_id
–contentB

Now, I want to create a many to many relationship table

table A_B
–tableA_id
–tableB_id

the question is that how to write a sql script (I am new to sql) to populate table A_B using the existing data from table A and table B. Many thanks,

Mark

Answer 1

If you want to populate table A_B, you’d have to do this:

INSERT INTO A_B (tableA_id, tableB_id)
SELECT A.ID, B.ID FROM A CROSS JOIN B

CROSS JOIN will relate each row in table A with each row in table B.

If you want to relate some rows in table A with some rows in table B, you need to be more specific, and do something like:

INSERT INTO A_B (tableA_id, tableB_id)
SELECT A.ID, B.ID FROM A INNER JOIN B
ON A.some_field = B.some_other_field