Il database MySQL interno si unisce all'utilizzo di più tabelle? sinistra sinistra join efficienza di utilizzo

mysql> use chenweiliang;
Database changed
mysql> SELECT * FROM tcount_tbl;
+---------------+--------------+
| chenweiliang_author | chenweiliang_count |
+---------------+--------------+
| 陈沩亮博客 | 10 |
| chenweiliang.com | 20 |
| Google | 22 |
+---------------+--------------+
3 rows in set (0.01 sec)
 
mysql> SELECT * from chenweiliang_tbl;
+-----------+---------------+---------------+-----------------+
| chenweiliang_id | chenweiliang_title | chenweiliang_author | submission_date |
+-----------+---------------+---------------+-----------------+
| 1 | 学习 PHP | 陈沩亮博客 | 2017-04-12 |
| 2 | 学习 MySQL | 陈沩亮博客 | 2017-04-12 |
| 3 | 学习 Java | chenweiliang.com | 2015-05-01 |
| 4 | 学习 Python | chenweiliang.com | 2016-03-06 |
| 5 | 学习 C | FK | 2017-04-05 |
+-----------+---------------+---------------+-----------------+
5 rows in set (0.01 sec)

mysql> SELECT a.chenweiliang_id, a.chenweiliang_author, b.chenweiliang_count FROM chenweiliang_tbl a INNER JOIN tcount_tbl b ON a.chenweiliang_author = b.chenweiliang_author;
+-------------+-----------------+----------------+
| a.chenweiliang_id | a.chenweiliang_author | b.chenweiliang_count |
+-------------+-----------------+----------------+
| 1 | 陈沩亮博客 | 10 |
| 2 | 陈沩亮博客 | 10 |
| 3 | chenweiliang.com | 20 |
| 4 | chenweiliang.com | 20 |
+-------------+-----------------+----------------+
4 rows in set (0.00 sec)

mysql> SELECT a.chenweiliang_id, a.chenweiliang_author, b.chenweiliang_count FROM chenweiliang_tbl a, tcount_tbl b WHERE a.chenweiliang_author = b.chenweiliang_author;
+-------------+-----------------+----------------+
| a.chenweiliang_id | a.chenweiliang_author | b.chenweiliang_count |
+-------------+-----------------+----------------+
| 1 | 陈沩亮博客 | 10 |
| 2 | 陈沩亮博客 | 10 |
| 3 | chenweiliang.com | 20 |
| 4 | chenweiliang.com | 20 |
+-------------+-----------------+----------------+
4 rows in set (0.01 sec)

mysql> SELECT a.chenweiliang_id, a.chenweiliang_author, b.chenweiliang_count FROM chenweiliang_tbl a LEFT JOIN tcount_tbl b ON a.chenweiliang_author = b.chenweiliang_author;
+-------------+-----------------+----------------+
| a.chenweiliang_id | a.chenweiliang_author | b.chenweiliang_count |
+-------------+-----------------+----------------+
| 1 | 陈沩亮博客 | 10 |
| 2 | 陈沩亮博客 | 10 |
| 3 | chenweiliang.com | 20 |
| 4 | chenweiliang.com | 20 |
| 5 | FK | NULL |
+-------------+-----------------+----------------+
5 rows in set (0.01 sec)

mysql> SELECT a.chenweiliang_id, a.chenweiliang_author, b.chenweiliang_count FROM chenweiliang_tbl a RIGHT JOIN tcount_tbl b ON a.chenweiliang_author = b.chenweiliang_author;
+-------------+-----------------+----------------+
| a.chenweiliang_id | a.chenweiliang_author | b.chenweiliang_count |
+-------------+-----------------+----------------+
| 1 | 陈沩亮博客 | 10 |
| 2 | 陈沩亮博客 | 10 |
| 3 | chenweiliang.com | 20 |
| 4 | chenweiliang.com | 20 |
| NULL | NULL | 22 |
+-------------+-----------------+----------------+
5 rows in set (0.01 sec)

<?
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.chenweiliang_id, a.chenweiliang_author, b.chenweiliang_count FROM chenweiliang_tbl a INNER JOIN tcount_tbl b ON a.chenweiliang_author = b.chenweiliang_author';
 
mysqli_select_db( $conn, 'chenweiliang' );
$retval = mysqli_query( $conn, $sql );
if(! $retval )
{
 die('无法读取数据: ' . mysqli_error($conn));
}
echo '<h2>陈沩亮博客 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> {$row['chenweiliang_id']}</td> ".
 "<td>{$row['chenweiliang_author']} </td> ".
 "<td>{$row['chenweiliang_count']} </td> ".
 "</tr>";
}
echo '</table>';
mysqli_close($conn);
?>