How do I know which table is left and right mysql

Question

I know the first select from query is the left table, but I am still confused when to use right and left join:

ex:

select chann.name from investments inv
right join channels chann on chann.id = inv.channel_id;


select chann.name from channels chann
left join investments inv on chann.id = inv.channel_id;

They both return the same result, why in first query is used right and why left on second one, Does this have to do with Foreign Key something?

Thanks

have you tried anything ? http://stackoverflow.com/questions/3308122/how-do-i-decide-when-to-use-right-joins-left-joins-or-inner-joins-or-how-to-dete http://stackoverflow.com/questions/5706437/whats-the-difference-between-inner-join-left-join-right-join-and-full-join — Kaushik, Apr 29 '15 at 13:54
A left join B is the same as B right join A. But humans usually find it much harder to understand right join. That's why most people use left join only. — jarlh, Apr 29 '15 at 13:57
This may help you to understand the difference in SQL joins: http://www.codeproject.com/Articles/33052/Visual-Representation-of-SQL-Joins — Fred, Apr 29 '15 at 13:58

score 3 · Accepted Answer · answered Apr 29 '15 at 13:55

3

In your example both query are the same, because when change right to left you also change tables order. For simple understanding this, consider follow example:

SELECT A.* FROM Table1 A LEFT JOIN Table2 B on A.key=B.Key

In this case, LEFT JOIN means all records from Table1, because is the table in the left. If you use right means all records from table 2

answered Apr 29 '15 at 13:55

ericpap

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What does "in the left" mean? Do you mean to the left of the join statement "LEFT JOIN"? To what does the positionality right or left relate? – fraxture Jun 28 '19 at 21:01
@fraxtureWhen you use `FROM TableA`, that's your starting point located on the LEFT, any other joined table that comes later is on the RIGHT... – Enissay Jul 29 '19 at 10:19

score 1 · Answer 2 · answered Apr 29 '15 at 14:03

LEFT JOIN: Return all rows from the left table, and the matched rows from the right table RIGHT JOIN: Return all rows from the right table, and the matched rows from the left table

Meaning that a left join will take all the records from your first table and join them only with the records on the right table that adhere to your ON condition A right join will do the oposite (take all from second table and join with only records on first table that adhere to ON condition)

How do I know which table is left and right mysql

2 Answers2