MYSQL 连接语句 JOIN
JOIN: 在两个或者多个表中查询数据，可以在 SELECT ,UPDAYE 和 DELETE 的语句中使用 MYSQL 的JOIN 联合多表查询。INNER HOINF(内连接，或等值链接） ： 获取两个表中字段匹配关系的记录。
LEFT JOIN （左连接 ） 获取左表中所有记录，即使右表没有队友的匹配记录。
RIGH 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。
先来个测试数据
mysql> use RUNOOB;
Database changed
mysql> SELECT * FROM tcount_tbl;
+---------------+--------------+
| runoob_author | runoob_count |
+---------------+--------------+
| 菜鸟教程| 10 |
| RUNOOB.COM | 20 |
| Google | 22 |
+---------------+--------------+
3 rows in set (0.01 sec)
mysql> SELECT * from runoob_tbl;
+-----------+---------------+---------------+-----------------+
| runoob_id | runoob_title| runoob_author | submission_date |
+-----------+---------------+---------------+-----------------+
| 1 | 学习 PHP | 菜鸟教程| 2017-04-12 |
| 2 | 学习 MySQL| 菜鸟教程| 2017-04-12 |
| 3 | 学习 Java | RUNOOB.COM | 2015-05-01 |
| 4 | 学习 Python | RUNOOB.COM | 2016-03-06 |
| 5 | 学习 C | FK | 2017-04-05 |
+-----------+---------------+---------------+-----------------+
5 rows in set (0.01 sec)
第一测： 使用 MYSQL 的 INNER JOIN(也可以省略 INNER 直接使用 JOIN 效果一样)连接两张表 来读取 RUNOOB_TBL表中所有 RUNOOB_AUTHOR 字段在 TCOUNT_TBL 表中对应的 runoob_count 字段值：
mysql> SELECT a.runoob_id, a.runoob_author, b.runoob_count FROM runoob_tbl a INNER JOIN tcount_tbl b ON a.runoob_author = b.runoob_author;
+-------------+-----------------+----------------+
| a.runoob_id | a.runoob_author | b.runoob_count |
+-------------+-----------------+----------------+
| 1 | 菜鸟教程 | 10 |
| 2 | 菜鸟教程 | 10 |
| 3 | RUNOOB.COM | 20 |
| 4 | RUNOOB.COM | 20 |
+-------------+-----------------+----------------+
4 rows in set (0.00 sec)就相当于 WHERE 语句
SELECT a.runoob_id, a.runoob_author, b.runoob_count FROM runoob_tbl a, tcount_tbl b WHERE a.runoob_author = b.runoob_author;
LEFT JOIN 与 JOIN 有所不同，前者会从左侧读取数据表的全部数据，即便右边无对应数据。
mysql> SELECT a.runoob_id, a.runoob_author, b.runoob_count FROM runoob_tbl a LEFT JOIN tcount_tbl b ON a.runoob_author = b.runoob_author;
+-------------+-----------------+----------------+
| a.runoob_id | a.runoob_author | b.runoob_count |
+-------------+-----------------+----------------+
| 1 | 菜鸟教程 | 10 |
| 2 | 菜鸟教程 | 10 |
| 3 | RUNOOB.COM | 20 |
| 4 | RUNOOB.COM | 20 |
| 5 | FK | NULL |
+-------------+-----------------+----------------+
5 rows in set (0.01 sec)
RIGHTJOIN 与 上述相反 为右侧全表
mysql> SELECT a.runoob_id, a.runoob_author, b.runoob_count FROM runoob_tbl a RIGHT JOIN tcount_tbl b ON a.runoob_author = b.runoob_author;
+-------------+-----------------+----------------+
| a.runoob_id | a.runoob_author | b.runoob_count |
+-------------+-----------------+----------------+
| 1 | 菜鸟教程 | 10 |
| 2 | 菜鸟教程 | 10 |
| 3 | RUNOOB.COM | 20 |
| 4 | RUNOOB.COM | 20 |
| NULL | NULL | 22 |
+-------------+-----------------+----------------+
5 rows in set (0.01 sec)
PHP中的用法：老套路 MYSQLI_QUERY()
<?php
$dbhost = 'localhost:3306';// mysql服务器主机地址
$dbuser = 'root'; // mysql用户名
$dbpass = '123456'; // mysql用户名密码
$conn = mysqli_connect($dbhost, $dbuser, $dbpass);
if(! $conn )
{
die('连接失败: ' . mysqli_error($conn));
}
// 设置编码，防止中文乱码
mysqli_query($conn , "set names utf8");
$sql = 'SELECT a.runoob_id, a.runoob_author, b.runoob_count FROM runoob_tbl a INNER JOIN tcount_tbl b ON a.runoob_author = b.runoob_author
mysqli_select_db( $conn, 'RUNOOB' );
$retval = mysqli_query( $conn, $sql );
if(! $retval )
{
die('无法读取数据: ' . mysqli_error($conn));
}
echo '<h2>菜鸟教程 MySQL JOIN 测试<h2>echo '<table border="1"><tr><td>教程 ID</td><td>作者</td><td>登陆次数</td></tr>while($row = mysqli_fetch_array($retval, MYSQL_ASSOC))
{
echo "<tr><td> {$row['runoob_id']}</td> ".
"<td>{$row['runoob_author']} </td> ".
"<td>{$row['runoob_count']} </td> ".
"</tr>";
}
echo '</table>mysqli_close($conn);
?>
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