Question

I have a SQL query that has 4 UNIONS and 4 LEFT JOINS. It is layed out as such:

SELECT ... FROM table1
    LEFT JOIN other_table1
UNION SELECT ... FROM table2
    LEFT JOIN other_table2
UNION SELECT ... other_table3
    LEFT JOIN other_table3
UNION SELECT ... FROM table4
    LEFT JOIN other_table4

Would it be better to run 4 separate queries and then merge the results with php after the fact? Or should I keep them together? Which would provide that fastest execution?

Answer 1

The most definitive answer is to test each method, however the UNION is most likely to be faster as only one query is run by MySQL as opposed to 4 for each part of the union.

You also remove the overhead of reading the data into memory in PHP and concatenating it. Instead, you can just do a while() or foreach() or whatever on one result.