How does Left Join / IS NULL eliminate records which are there in one table and not in the other?

Question

I am having a tough time to understand why does LEFT JOIN / IS NULL eliminate records which are there in one table and not in the other. Here is an example

SELECT  l.id, l.value
FROM    t_left l
LEFT JOIN t_right r
ON      r.value = l.value
WHERE   r.value IS NULL

Why should r.value = NULL eliminate records ? I am not understanding . I know I am missing something very basic but at present I cant figure out even that basic one. I would appreciate if someone explains it to me in detail .

I want a very basic explanation.

Answer 1

This could be explained with the following

mysql> select * from table1 ;
+------+------+
| id   | val  |
+------+------+
|    1 |   10 |
|    2 |   30 |
|    3 |   40 |
+------+------+
3 rows in set (0.00 sec)

mysql> select * from table2 ;
+------+------+
| id   | t1id |
+------+------+
|    1 |    1 |
|    2 |    2 |
+------+------+
2 rows in set (0.00 sec)

Here table1.id <-> table2.t1id

Now when we do a left join with the joining key and if the left table is table1 then it will get all the data from table1 and in non-matching record on table2 will be set to null

mysql> select t1.* , t2.t1id from table1 t1 
left join table2 t2 on t2.t1id = t1.id ;
+------+------+------+
| id   | val  | t1id |
+------+------+------+
|    1 |   10 |    1 |
|    2 |   30 |    2 |
|    3 |   40 | NULL |
+------+------+------+

3 rows in set (0.00 sec)

See that table1.id = 3 does not have a value in table2 so its set as null When you apply the where condition it will do further filtering

mysql> select t1.* , t2.t1id from table1 t1 
left join table2 t2 on t2.t1id = t1.id where t2.t1id is null;
+------+------+------+
| id   | val  | t1id |
+------+------+------+
|    3 |   40 | NULL |
+------+------+------+
1 row in set (0.00 sec)

Answer 2

Let's assume r-table is employees, and r_table is computers. Some employees don't have computers. Some computers are not assigned to anyone yet.

1. Inner join:

   SELECT  l.*, r.*
   FROM    employees l
   JOIN computers r
   ON      r.id = l.comp_id
   

   gives you the list of all employees who HAVE a computer, and the info about computer assigned for each of them. The employees without a computer will NOT appear on this list.

2. Left join:

   SELECT  l.*, r.*
   FROM    employees l
   LEFT JOIN computers r
   ON      r.id = l.comp_id
   

   gives you the list of ALL employees. The employees with computer will show the computer info. The employees without computers will appear with NULLs instead of computer info.

3. Finally

   SELECT  l.*, r.*
   FROM    employees l
   LEFT JOIN computers r
   ON      r.id = l.comp_id
   WHERE   r.id IS NULL
   

   Left join with the WHERE clause will start with the same list as the left join (2), but then it will keep only those employees that do not have corresponding information in the computer table, that is, employees without the computers.

   I this case, selecting anything from the r table will be just nulls, so you can leave those fields out and select only stuff from the l table:

   SELECT  l.*
   FROM ...
   

Try this sequence of selects and observe the output. Each next step builds on the previous one.

Please let me know if this explanation is understandable, or you'd like me to elaborate some more.

EDITED TO ADD: Here's sample code to create the two tables used above:

CREATE TABLE employees
(  id INT NOT NULL PRIMARY KEY,
   name VARCHAR(20),
   comp_id INT);

INSERT INTO employees (id, name, comp_id) VALUES (1, 'Becky', 1);
INSERT INTO employees (id, name, comp_id) VALUES (2, 'Anne', 7);
INSERT INTO employees (id, name, comp_id) VALUES (3, 'John', 3);
INSERT INTO employees (id, name) VALUES (4, 'Bob');

CREATE TABLE computers
(  id INT NOT NULL PRIMARY KEY,
   os VARCHAR(20) );

INSERT INTO computers (id, os) VALUES (1,'Windows 7');
INSERT INTO computers (id, os) VALUES (2,'Windows XP');
INSERT INTO computers (id, os) VALUES (3,'Unix');
INSERT INTO computers (id, os) VALUES (4,'Windows 7');

There are 4 employees. Becky and John have computers. Anne and Bob do not have a computer. (Anne, has a comp_id 7, which doesn't correspond to any row in computers table - so, she doesn't really have a computer.)