mysql join select explained

Question

I have two tables: 'posts' and 'users' every post has a 'ref_id' column to get the user id who posted it.

Now, I am getting posts this way:

$this->db->query("SELECT * FROM posts WHERE time > '$timeLimit' LIMIT 50");

I can't understand how to join every result to get the poster related data as well. What I am doing right now is basically a loop inside a loop, where foreach of the result, get their user info. But it is pretty obvious that this is very wrong,

Apparently I need to start using joins, but how does one do it? this should be a really simple example to work with, I suppose.

Any help? Thank you.

Answer 1

SELECT posts.*, users.* 
FROM posts 
INNER JOIN users 
  ON posts.posted_by = users.id;

Answer 2

Like this:

SELECT
  posts.*,,
  users.Username
FROM posts
INNER JOIN users ON posts.ref_id = users.user_id;

---

Explanation:

To JOIN to any tables with each others, there are two things; the JOIN type and the join condition. There are three main types of join:

• INNER JOIN , only the rows that match the join condition will be returned from the two tables no more rows. But:

• LEFT OUTER JOIN , when you join two tables you will have one on the left of the join keyword and the other one will be in the right:

  FROM Table1 <------------- This is the left table. LEFT OUTER JOIN table2 .... <------------- This is the right table.

In LEFT OUTER JOIN the unmatched rows from the left table will be included in the result set.

• RIGHT OUTER JOIN the unmatched rows from the right table will be included in the result set.

• CROSS JOIN this will perform a Cartesian product from the two tables.

In our query, the query will reutrn all the users from the users table only if the ref_id equal to the user_id column form the posts table.

For more information and explanations:

• A Visual Explanation of SQL Joins .

• Another Visual Representation of SQL Joins .

• Join syntax in MySQL

Answer 3

SELECT user.name
FROM users
INNER JOIN posts
ON posts.ref_id == user.id
AND posts.time > 50

http://www.w3schools.com/sql/sql_join_inner.asp