Question

I was wondering how exactly inner joins works in mysql.

If I do

SELECT * FROM A a 
INNER JOIN B b ON a.row = b.row
INNER JOIN C c ON c.row2 = b.row2
WHERE name='Paul';

Does it do the joins first, then pick the ones where name = paul? Because when I do it this way, it is SUPER DUPER slow.

is there a way to do something along the lines:

SELECT * FROM (A a WHERE name='paul')
INNER JOIN B b ON a.row = b.row
INNER JOIN C c ON c.row2 = b.row2]

When I try it that way, I just get an error.

or alternately, is it better to just have 3 separate queries, one for A, B and C? example:

string query1 = "SELECT * FROM A WHERE name = 'paul'";
//send query, get data reader
string query2 = "SELECT * FROM b WHERE b = " + query1.b;
//send query, get data reader
string query3 = "SELECT * FROM C WHERE c = " + query1.c;
//send query, get data reader

Obviously this is just pseudo code, but I think it illustrates the point.

Which way is faster/recommended?

Edit
Table structure:

**tblTimesheet**
int timesheetID (primary key)
datetime date
varchar username
int projectID
string description
float hours

**tblProjects**
int projectID (primary key)
string project name
int clientID

**tblClients**
int clientID
string clientName

The join that I want is:

select * from tblTimesheet time
INNER JOIN tblProject proj on time.projectID = proj.projectID
INNER JOIN tblClient client on proj.clientID = client.clientID
WHERE username = 'paul';

something like that

Answer 1

You are probably missing an index on a key table; you can use the MySql EXPLAIN keyword to help in finding out where your query is slow.

To answer another section of your question;

is there a way to do something along the lines:

SELECT * FROM (A a WHERE name='paul')
INNER JOIN B b ON a.row = b.row
INNER JOIN C c ON c.row2 = b.row2]

You can use a SubQuery;

SELECT * 
FROM (SELECT * FROM  tblTimesheet WHERE username = 'Paul') AS time
INNER JOIN tblProject proj on time.projectID = proj.projectID
INNER JOIN tblClient client on proj.clientID = client.clientID

What this query is effectively doing is attempting to prefilter the fields the JOIN will operate on. Rather than join all the fields to together, and then filter those down, it only attempts to JOIN fields from tblTimesheet where the name is ‘Paul’ first.

However, the query optimizer should already be doing this so this query should perform similarly to your original query.

For more help with indexes, the understanding of which will aid you greatly in database development, start by looking at a tutorial like this one.