MySQL 连接的使用

在前几章节中，我们已经学会了如果在一张表中读取数据，这是相对简单的，但是在真正的应用中经常需要从多个数据表中读取数据。

本章节我们将向大家介绍如何使用 MySQL 的 JOIN 在两个或多个表中查询数据。

你可以在SELECT, UPDATE 和 DELETE 语句中使用 Mysql 的 JOIN 来联合多表查询。

JOIN 按照功能大致分为如下三类：

• INNER JOIN（内连接,或等值连接）：获取两个表中字段匹配关系的记录。
• LEFT JOIN（左连接）：获取左表所有记录，即使右表没有对应匹配的记录。
• RIGHT JOIN（右连接）： 与 LEFT JOIN 相反，用于获取右表所有记录，即使左表没有对应匹配的记录。

本章节使用的数据库结构及数据下载：W3CSCHOOL.sql。

---

在命令提示符中使用 INNER JOIN

我们在W3CSCHOOL数据库中有两张表 tcount_tbl 和 zijiebao_tbl。两张数据表数据如下：

实例

尝试以下实例：

root@host# mysql -u root -p password;
Enter password:*******
mysql> use W3CSCHOOL;
Database changed
mysql> SELECT * FROM tcount_tbl;
+-----------------+----------------+
| zijiebao_author | zijiebao_count |
+-----------------+----------------+
| mahran          |             20 |
| mahnaz          |           NULL |
| Jen             |           NULL |
| Gill            |             20 |
| John Poul       |              1 |
| Sanjay          |              1 |
+-----------------+----------------+
6 rows in set (0.01 sec)
mysql> SELECT * from zijiebao_tbl;
+-------------+----------------+-----------------+-----------------+
| zijiebao_id | zijiebao_title | zijiebao_author | submission_date |
+-------------+----------------+-----------------+-----------------+
|           1 | Learn PHP      | John Poul       | 2007-05-24      |
|           2 | Learn MySQL    | Abdul S         | 2007-05-24      |
|           3 | JAVA Tutorial  | Sanjay          | 2007-05-06      |
+-------------+----------------+-----------------+-----------------+
3 rows in set (0.00 sec)
mysql>

接下来我们就使用MySQL的INNER JOIN(也可以省略 INNER 使用 JOIN，效果一样)来连接以上两张表来读取zijiebao_tbl表中所有zijiebao_author字段在tcount_tbl表对应的zijiebao_count字段值：

mysql> SELECT a.zijiebao_id, a.zijiebao_author, b.zijiebao_count FROM zijiebao_tbl a INNER JOIN tcount_tbl b ON a.zijiebao_author = b.zijiebao_author;
+-----------+---------------+--------------+
| zijiebao_id | zijiebao_author | zijiebao_count |
+-----------+---------------+--------------+
|         1 | John Poul     |            1 |
|         3 | Sanjay        |            1 |
+-----------+---------------+--------------+
2 rows in set (0.00 sec)

以上 SQL 语句等价于：

mysql> SELECT a.zijiebao_id, a.zijiebao_author, b.zijiebao_count FROM zijiebao_tbl a, tcount_tbl b WHERE a.zijiebao_author = b.zijiebao_author;
+-------------+-----------------+----------------+
| zijiebao_id | zijiebao_author | zijiebao_count |
+-------------+-----------------+----------------+
|           1 | John Poul       |              1 |
|           3 | Sanjay          |              1 |
+-------------+-----------------+----------------+
2 rows in set (0.01 sec)
mysql>

---

MySQL LEFT JOIN

MySQL left join 与 join 有所不同。 MySQL LEFT JOIN 会读取左边数据表的全部数据，即便右边表无对应数据。

实例

尝试以下实例，以 zijiebao_tbl 为左表，tcount_tbl 为右表，理解MySQL LEFT JOIN的应用：

root@host# mysql -u root -p password;
Enter password:*******
mysql> use W3CSCHOOL;
Database changed
mysql> SELECT a.zijiebao_id, a.zijiebao_author, b.zijiebao_count FROM zijiebao_tbl a LEFT JOIN tcount_tbl b ON a.zijiebao_author = b.zijiebao_author;
+-------------+-----------------+----------------+
| zijiebao_id | zijiebao_author | zijiebao_count |
+-------------+-----------------+----------------+
|           1 | John Poul       |              1 |
|           2 | Abdul S         |           NULL |
|           3 | Sanjay          |              1 |
+-------------+-----------------+----------------+
3 rows in set (0.02 sec)

以上实例中使用了LEFT JOIN，该语句会读取左边的数据表zijiebao_tbl的所有选取的字段数据，即便在右侧表tcount_tbl中没有对应的zijiebao_author字段值。

---

MySQL RIGHT JOIN

MySQL RIGHT JOIN 会读取右边数据表的全部数据，即便左边边表无对应数据。

实例

尝试以下实例，以 tcount_tbl 为左表，zijiebao_tbl 为右表，理解MySQL RIGHT JOIN的应用：

root@host# mysql -u root -p password;
Enter password:*******
mysql> use W3CSCHOOL;
Database changed
mysql> SELECT b.zijiebao_id, b.zijiebao_author, a.zijiebao_count FROM tcount_tbl a RIGHT JOIN zijiebao_tbl b ON a.zijiebao_author = b.zijiebao_author;
+-------------+-----------------+----------------+
| zijiebao_id | zijiebao_author | zijiebao_count |
+-------------+-----------------+----------------+
|           1 | John Poul       |              1 |
|           2 | Abdul S         |           NULL |
|           3 | Sanjay          |              1 |
+-------------+-----------------+----------------+
3 rows in set (0.02 sec)

以上实例中使用了 RIGHT JOIN，该语句会读取右边的数据表 zijiebao_tbl 的所有选取的字段数据，即便在左侧表tcount_tbl中没有对应的zijiebao_author字段值。

---

在PHP脚本中使用JOIN

PHP 中使用mysql_query()函数来执行SQL语句，你可以使用以上的相同的SQL语句作为mysql_query()函数的参数。

尝试如下实例:

<?php
$dbhost = 'localhost:3036';
$dbuser = 'root';
$dbpass = 'rootpassword';
$conn = mysql_connect($dbhost, $dbuser, $dbpass);
if(! $conn )
{
  die('Could not connect: ' . mysql_error());
}
$sql = 'SELECT a.zijiebao_id, a.zijiebao_author, b.zijiebao_count FROM zijiebao_tbl a INNER JOIN tcount_tbl b ON a.zijiebao_author = b.zijiebao_author';

mysql_select_db('W3CSCHOOL');
$retval = mysql_query( $sql, $conn );
if(! $retval )
{
  die('Could not get data: ' . mysql_error());
}
while($row = mysql_fetch_array($retval, MYSQL_ASSOC))
{
    echo "Author:{$row['zijiebao_author']}  <br> ".
         "Count: {$row['zijiebao_count']} <br> ".
         "Tutorial ID: {$row['zijiebao_id']} <br> ".
         "--------------------------------<br>";
} 
echo "Fetched data successfully\n";
mysql_close($conn);
?>