Question

Guys, I have two basic tables (id, first_name).

I want to run table 1 against table 2, delete duplicates, and spit out the cleansed version of table 1.

What’s the easiest way to do this with PHP/MySQL?

Thanks.

Answer 1

This will delete all records from t1 that also exist in t2, leaving you with a stripped-down t1, but possibly with records in t2 that don’t exist in t1:

DELETE FROM table1 t1
WHERE t1.id IN
    (SELECT id from table2 t2
        WHERE t2.id = t1.id AND t2.first_name = t1.first_name
    )

*You may want to consider using EXISTS instead of IN, as per Brian Hooper’s suggestion.

This will combine the two tables into a third table (t3), removing duplicates along the way:

SELECT * INTO t3 
FROM t1 UNION SELECT * FROM t2

That will work for SQL Server (definitely) and MySQL (I think) but MySQL supports CREATE TABLE table_name AS select-statement, so you could use:

CREATE TABLE t3 AS
(SELECT * FROM t1 UNION DISTINCT SELECT * FROM t2)

*The DISTINCT keyword is optional – it’s the default behaviour