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MySQL INNER JOIN

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What is INNER JOIN in MySQL?

In MySQL the INNER JOIN selects all rows from both participating tables to appear in the result if and only if both tables meet the conditions specified in the ON clause. JOIN, CROSS JOIN, and INNER JOIN are syntactic equivalents. In standard SQL, they are not equivalent. INNER JOIN is used with an ON clause, CROSS JOIN is used otherwise.

Pictorial presentation of MySQL INNER JOIN :

Pictorial presentation of MySQL INNER JOIN

MySQL INNER JOIN Syntax :

MySQL supports the following JOIN syntaxes for the table_references (A table reference is also known as a join expression.) part of SELECT statements and multiple-table UPDATE and DELETE statements :

table_references:
    escaped_table_reference [, escaped_table_reference] ...

escaped_table_reference:
    table_reference
  | { OJ table_reference }

table_reference:
    table_factor
  | join_table

table_factor:
    tbl_name [PARTITION (partition_names)] 
        [[AS] alias] [index_hint_list]
  | table_subquery [AS] alias
  | ( table_references )

join_table:
    table_reference [INNER | CROSS] JOIN table_factor [join_condition]
  | table_reference STRAIGHT_JOIN table_factor
  | table_reference STRAIGHT_JOIN table_factor ON conditional_expr
  | table_reference {LEFT|RIGHT} [OUTER] JOIN table_reference join_condition
  | table_reference NATURAL [{LEFT|RIGHT} [OUTER]] JOIN table_factor

join_condition:
    ON conditional_expr
  | USING (column_list)

index_hint_list:
    index_hint [, index_hint] ...

index_hint:
    USE {INDEX|KEY}
      [FOR {JOIN|ORDER BY|GROUP BY}] ([index_list])
  | IGNORE {INDEX|KEY}
      [FOR {JOIN|ORDER BY|GROUP BY}] (index_list)
  | FORCE {INDEX|KEY}
      [FOR {JOIN|ORDER BY|GROUP BY}] (index_list)

index_list:
    index_name [, index_name] ...

Example : MySQL INNER JOIN

When combining records from more than one tables, an user needs to indicate how records in a table can be matched to records in the other. As the both of tables have a cate_id column, we can match using that column. The ON clause is used to match records in two tables, based on the value of cate_id column. Usage of INNER JOIN combines the tables. An INNER JOIN allows rows from either table to appear in the result if and only if both tables meet the conditions specified in the ON clause.

In this example, the ON clause specifies that the cate_id column of both book_mast and category table must match. If a cate_id does not appear in both of the tables, the row will not appear in the result because the condition in the ON clause fails. Only those categories will participate in the JOIN whose books are written in ENGLISH.

SELECT book_mast.book_id,book_mast.book_name,cate_descrip           
FROM book_mast 
INNER JOIN category          
ON book_mast.cate_id=category.cate_id        
WHERE book_mast.pub_lang="English";

Sample table : book_mast

Sample table : category :

Output :

mysql> SELECT book_mast.book_id,book_mast.book_name,cate_descrip           
    -> FROM book_mast 
    -> INNER JOIN category          
    -> ON book_mast.cate_id=category.cate_id        
    -> WHERE book_mast.pub_lang="English";
+---------+-------------------------------------+--------------+
| book_id | book_name                           | cate_descrip |
+---------+-------------------------------------+--------------+
| BK001   | Introduction to Electrodynamics     | Science      | 
| BK002   | Understanding of Steel Construction | Technology   | 
| BK004   | Transfer  of Heat and Mass          | Technology   | 
| BK010   | Fundamentals of Thermodynamics      | Technology   | 
| BK012   | The Nature  of World                | Nature       | 
| BK009   | Mental Health Nursing               | Medical      | 
+---------+-------------------------------------+--------------+
6 rows in set (0.04 sec)

PHP script

<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>example-inner-join-with-multiple-tables php mysql examples | w3resource</title>
<meta name="description" content="example-inner-join-with-multiple-tables php mysql examples | w3resource">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">
</head>
<body>
<div class="container">
<div class="row">
<div class="col-md-12">
<h2>List of the book ids, name of the book and category description:</h2>
<table class='table table-bordered'>
<tr>
<th>Book ID</th><th>Name of the book</th><th>Category description</th>
</tr>
<?php
$hostname="your_hostname";
$username="your_username";
$password="your_password";
$db = "your_dbname";
$dbh = new PDO("mysql:host=$hostname;dbname=$db", $username, $password);
foreach($dbh->query('SELECT book_mast.book_id,book_mast.book_name,cate_descrip
FROM book_mast
INNER JOIN category
ON book_mast.cate_id=category.cate_id
WHERE book_mast.pub_lang="English"') as $row) {
echo "<tr>";
echo "<td>" . $row['book_id'] . "</td>";
echo "<td>" . $row['book_name'] . "</td>";
echo "<td>" . $row['cate_descrip'] . "</td>"; 
echo "</tr>";
}
?>
</tbody></table>
</div>
</div>
</div>
</body>
</html>

View the example in browser

Example : MySQL INNER JOIN with alias

The following MySQL statement returns book ids, the name of the the book, publisher's id, category description and publisher's language, where publisher's language is English and publisher's id is not equal to P004. Notice that aliases have been used to refer the column names. An INNER JOIN is performed based upon the condition that a category id in book_mast table must exist in category table also.

SELECT bk.book_id,bk.book_name,bk.pub_id,ca.cate_descrip,bk.pub_lang 
FROM book_mast AS bk
INNER JOIN category AS ca  ON bk.cate_id=ca.cate_id  AND
bk.pub_lang="English" AND bk.pub_id<>'P004';

Sample table : book_mast :

Output :

mysql> SELECT bk.book_id,bk.book_name,bk.pub_id,ca.cate_descrip,bk.pub_lang 
    -> FROM book_mast AS bk
    -> INNER JOIN category AS ca  ON bk.cate_id=ca.cate_id  AND
    -> bk.pub_lang='English' AND bk.pub_id<>'P004';
+---------+-------------------------------------+--------+--------------+----------+
| book_id | book_name                           | pub_id | cate_descrip | pub_lang |
+---------+-------------------------------------+--------+--------------+----------+
| BK001   | Introduction to Electrodynamics     | P003   | Science      | English  | 
| BK002   | Understanding of Steel Construction | P001   | Technology   | English  | 
| BK010   | Fundamentals of Thermodynamics      | P007   | Technology   | English  | 
| BK012   | The Nature  of World                | P008   | Nature       | English  | 
| BK009   | Mental Health Nursing               | P007   | Medical      | English  | 
+---------+-------------------------------------+--------+--------------+----------+
5 rows in set (0.00 sec)

MySQL INNER JOIN using three tables

Sample tables :

table - doctors
+-------+---------+
| docid | dname   |
+-------+---------+
|     1 | A.VARMA | 
|     2 | D.GOMES | 
+-------+---------+


table - specialize
+------+----------+-------+
| spid | desc     | docid |
+------+----------+-------+
|    1 | special1 |     1 | 
|    2 | special2 |     2 | 
+------+----------+-------+


table - timeschedule
+-----+------+----------+-------+
| tid | tday | sit_time | docid |
+-----+------+----------+-------+
|   1 | MON  | 17:00:00 |     1 | 
|   2 | WED  | 08:00:00 |     1 | 
|   3 | TUE  | 16:00:00 |     2 | 
|   4 | FRI  | 09:00:00 |     2 | 
+-----+------+----------+-------+

The above tables are related to each other. In doctors, specialize and timeschedule tables the docid, spid and tid are primary key consecutively. The docid in specialize table and timeschedule tables are a foreign key, which is the reference to primary key docid of doctors table.

If we want all records for a doctor who are specialized in special1 and seat in his chamber on Wednesday (WED) in his schedule time, the following SQL can be used-

SELECT a.docid,a.dname,
       b.desc,c.tday,c.sit_time
  FROM doctors a 
    INNER JOIN specialize b
     ON a.docid=b.docid
    INNER JOIN timeschedule c
     ON a.docid=c.docid
        WHERE a.docid=1 AND c.tday='WED';

Output :

+-------+---------+----------+------+----------+
| docid | dname   | desc     | tday | sit_time |
+-------+---------+----------+------+----------+
|     1 | A.VARMA | special1 | WED  | 08:00:00 | 
+-------+---------+----------+------+----------+

Explanation :

Step-1

SELECT a.docid,a.dname,b.desc
FROM doctors a 
INNER JOIN specialize b
ON a.docid=b.docid;

MySQL INNER JOIN using three tables - Part-1

Step-2

SELECT a.docid,a.dname,
b.desc,c.tday,c.sit_time
FROM doctors a 
INNER JOIN specialize b
ON a.docid=b.docid
INNER JOIN timeschedule c
ON a.docid=c.docid;

MySQL INNER JOIN using three tables - Part-2

Step-3

SELECT a.docid,a.dname,
b.desc,c.tday,c.sit_time
FROM doctors a 
INNER JOIN specialize b
ON a.docid=b.docid
INNER JOIN timeschedule c
ON a.docid=c.docid
WHERE a.docid=1 AND c.tday='WED';

MySQL INNER JOIN using three tables - Part-3

Key points to remember

Click on the following to get the slides presentation -

MySQL JOINS, slide presentation

INNER JOINS: SQL and other Relational Databases



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