Shantanu's Blog

Corporate Consultant

June 24, 2005

 

MySQL Case Study - 31

I'm looking for a query to return values which occur in one table, but not in the other. Say we have two tables (table_1, table_2) each with a column titled "appointments". I want a list of results for appointments which are in table_1, but not in table_2. I have tried the following, but all I get is a huge list of duplicate appointments which occur in both tables:

SELECT table_1.appointments
FROM table_1, table_2
WHERE table_1.appointments<>table_2.appointments;

I've also tried 'NOT IN' for this but the result was just the same.

Comments:
You can use a LEFT OUTER JOIN:

SELECT table_1.appointments
FROM table_1
LEFT JOIN table_2 ON table_2.appointments = table_1.appointments
WHERE table_2.appointments IS NULL

This query joins table_1 with table_2, taking all rows from table_1 and only those rows from table_2 for which the ON condition is true. If there is no match in table_2, it returns NULL for the columns of table_2. The WHERE clause therefore makes sure that only those rows are returned where there is no match in table_2.

It's also the perfect place to use a sub-query (if your using version 4.1 of MySQL).

SELECT table_1.appointments
FROM table_1
WHERE table_1.appointments NOT IN (select
table_2.appointments from table_2);

It's possible depending on the number of records in both table1 and table2 that a subquery will be faster than the join.
 
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