SQL Select on multiple tables to check if value exists in one table and not used in other

Question

I have 2 tables:

table1 (id,usedcode)
table2 (codeid,uniquecode)

I want to be able to check if a certain value exists in uniquecode of Table2, but is not already used in Table1

possible duplicate of [How can I query using a foreign key in MySQL?](http://stackoverflow.com/questions/12483109/how-can-i-query-using-a-foreign-key-in-mysql) — Charles Forest, Feb 08 '15 at 14:27
What exactly are you trying to select if this condition holds? — Kacy, Feb 08 '15 at 14:32

score 1 · Answer 1 · answered Feb 08 '15 at 14:36

1

Try using left join as below:

SELECT t2.*
FROM table2 t2 LEFT JOIN table1 t1
ON t2.uniquecode = t1.usedcode
WHERE t1.usedcode IS null

answered Feb 08 '15 at 14:36

SMA

score 0 · Accepted Answer · answered Feb 08 '15 at 14:25

0

SELECT uniquecode FROM Table2
WHERE NOT EXISTS( 
    SELECT * FROM Table1 WHERE usedcode = uniquecode
)

In English the query is saying, "Select all unique codes from table 2 that don't exist in table 1 as a usedcode".

answered Feb 08 '15 at 14:25

Kacy

2 Answers2