Question

I wonder if it’s possible to make a query ignore a table if there is no match in that table but shows matches from the other tables?
Let’s say I have three tables and I use a query like this:

SELECT table1.username, table2.age, table3.something_else FROM table1, table2, table3 
WHERE (table1.id = 37)
AND (table2.id = table1.id)
AND (table3.id = table1.id)

So this works if the id exists in all tables but if the id does not exist in one of the tables the query would’nt return any values at all. How would I write the query if I would return the columns where that id does exists?

Answer 1

use a LEFT JOIN

For instance

SELECT table1.username, table2.age, table3.something_else
FROM table1
LEFT JOIN table2 ON (table2.id = table1.id)
LEFT JOIN table3 ON (table3.id = table1.id)
WHERE (table1.id = 37)

When you include additional tables using a comma you are doing an implicit JOIN but it is an inner join.

Read up on JOINs here

In the mean time here’s a nice summary image: