SQL left join query
A report can be generated by collecting student details from Student master table and the semester and marks from the examination result table.
Let us understand the difference between LEFT join and WHERE condition.
There are two tables with some sample data. We will apply first sql where command to these tables.
| Table one ( t1 ) | Table two ( t2) | ||||||||||||||||
|
|
Joining tables Using SQL LEFT Join, RIGHT Join and Inner Join
Using WHERE Query
SELECT t1.id,t1.name1 FROM `t1`,t2 WHERE t1.id=t2.id
The output of the above SQL command will give records of MySQL table t1 for which
there is a record present in table t2 and linking is done by id field.
| id | name1 |
|---|---|
| 1 | one1 |
| 2 | two1 |
USING LEFT JOIN
SELECT t1.id, name1, t2.id, t2.name2 FROM t1
LEFT JOIN t2 ON t1.id = t2.id
We can display all the records of table t1 along with the (matching )
records of t2 if matching is found by linking them through ID field. Here is the left join SQL to display this and below that the output is listed. | id | name1 | id | name2 |
|---|---|---|---|
| 1 | one1 | 1 | one2 |
| 2 | two1 | 2 | two2 |
| 3 | three1 | NULL | NULL |
LEFT JOIN with IS NULL
The above result shows a simple left join and its output. We may be interested to identify the records in table t1 for which there is no matching record present in table t2. Here we have to use left join and link the tables by id field. Here is the query using left join and the output is below that.
SELECT t1.id, name1 FROM t1 LEFT JOIN t2 on t1.id = t2.id
WHERE ISNULL(t2.id)SELECT t1.id, name1 FROM t1 LEFT JOIN t2 on t1.id = t2.id
WHERE (t2.id) is null| id | name1 |
|---|---|
| 3 | three1 |
In the above result we have seen how the output works when we have to get records which are not present in other table. Now let us find out the other way , we will get the record of table one for which matching record exists in table 2. Here is the left join sql and below that the results of the query.
SELECT t1.id,name1,t2.id,t2.name2 FROM t1
LEFT JOIN t2 ON t1.id = t2.id WHERE NOT ISNULL( t2.id )SELECT t1.id,name1,t2.id,t2.name2 FROM t1
LEFT JOIN t2 ON t1.id = t2.id WHERE ( t2.id ) NOT is null
|
PHP script using left join to display records
We will try to develop one PHP script to display records by using left join query. Here we have used PHP PDO connection to connect to MySQL database. You can read more on PHP connection to MySQL using PDO here.After connection we have used foreach loop to display all the records. Here is the code.
<?Php
require "config.php"; // Database Connection details
$count="SELECT t1.id, name1 FROM t1 LEFT JOIN t2 on t1.id = t2.id
WHERE (t2.id) is null";
echo "<table>";
echo "<tr><th>id</th><th>name1</th></tr>";
foreach ($dbo->query($count) as $row) {
echo "<tr ><td>$row[id]</td><td>$row[name1]</td></tr>";
}
echo "</table>";
?>
Here is the SQL dump of two tables t1 & t2
Application of LEFT Join Query
We have two tables, one is the student list and the other one is a selected football team. Some of the students are selected to play for the football team. In our football table, we have kept two student ids (selected for the football team ).Using left join we can list out various way to display the list.
- Display all students with a mark for those who are selected
SELECT * FROM `student` LEFT JOIN student_football ON id=f_id
From the above list you can see by using WHERE condition we can filter out our required records
SELECT * FROM `student` LEFT JOIN student_football
ON id=f_id WHERE f_id IS NOT NULL
SELECT * FROM `student` LEFT JOIN student_football
ON id=f_id WHERE f_id IS NULLUsing three tables in LEFT Join Query
In above example we have seen how to find out records present in one table and not present in another table. Let us try for another example.In addition to football team there is one more baseball team. Two students are selected for baseball team. So now we have two in football team and two in baseball team.
Display all students with a mark for those who are selected for teams ( any )
SELECT * FROM `student`
LEFT JOIN student_football ON id=f_id
LEFT JOIN student_baseball on id=b_id
Next, let us find out who are the students not selected in any of the team. In other words, find out the records in student table that don't have matching records in football or baseball table ( team )
SELECT * FROM `student`
LEFT JOIN student_football ON id=f_id
LEFT JOIN student_baseball on id=b_id
WHERE f_id IS NULL and b_id is NULLYou can read more Null data here .
How aggregate data from one table is used to update another table by using LEFT Join
Download sql dump of student and student_footbal tables
.
For better understanding of LEFT JOIN , try these exercises
Exercise I Sample code for generating reports on Sales.
Exercise II Sample code for generating Product Stock and Price .
CROSS JOIN
LEFT JOIN using Multiple Tables
Exercise I Sample code for generating reports on Sales.
Exercise II Sample code for generating Product Stock and Price .
RIGHT JOIN LINKING TABLES Union of two tables and more Joining the table to itself by inner join
Linking of more than one table to get matching records Sub Queries to get records with IN and ANY Exercise : Sales - Agent using table JOIN and Date functions
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| Valerie | 21-11-2011 |
| This was very helpful, thanks. Your choice of table and field names made it easy to understand what you were doing. | |
| ramesh | 24-01-2012 |
| short and sweet...... tanks for ur info...... | |
| Bakht azam | 19-04-2012 |
| so grate notes i got more information from your site thankx | |
| Langat | 18-10-2013 |
| This was very helpful me, thanks now I can link tables in MYSQL | |
| Jitender | 21-04-2015 |
| Thanks a lot, you solved my problem. | |
| teaser141 | 04-02-2016 |
| Thanks. This article were helpful :-) | |
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