Q

MySQL full outer join workaround

A MySQL expert offers a workaround for the lack of syntax support for full outer joins.

Is it possible to do a joint query for 2 tables in MySQL to compare the fields, whether the data used is the same in both the tables or not?
Yes. The easiest way assumes that you have some ID field that should match in both tables. You can then join the tables on that ID and compare the other fields in the table for differences:
SELECT *
FROM
  table1
  JOIN table2 ON table1.id = table2.id 
WHERE
  (table1.field1, table1.field2, table1.field3) != (table2.field1, table2.field2, table3.field3)

If the tables have no ID field in common, you might need a full outer join to find the differences between the tables. MySQL doesn't offer syntax for a full outer join, but you can implement one using the union of a left and a right join. Since no indexes are likely to be used, expect for these results to take a long time on tables of any significant size.

SELECT *
FROM
  table1
  LEFT JOIN table2 ON (table1.field1, table1.field2, table1.field3) = (table2.field1, table2.field2, table3.field3)
WHERE
  table2.field1 IS NULL
UNION
SELECT *
FROM
  table1
  RIGHT JOIN table2 ON (table1.field1, table1.field2, table1.field3) = (table2.field1, table2.field2, table3.field3)
WHERE
  table1.field1 IS NULL
This was first published in March 2007

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