This article brings you relevant knowledge about MySQL, which mainly introduces the use of MySQL WHERE clause. Friends who are interested can take a look at it together. I hope it will be helpful to everyone.
We know that we use the SQL SELECT statement to read data from a MySQL table.
To conditionally select data from a table, add a WHERE clause to the SELECT statement.
The following is the general syntax for the SQL SELECT statement to read data from the data table using the WHERE clause:
SELECT field1, field2,...fieldN FROM table_name1, table_name2... [WHERE condition1 [AND [OR]] condition2.....
The following is a list of operators that can be used in the WHERE clause.
The examples in the following table assume that A is 10 and B is 20
Description | Example | |
---|---|---|
Equal sign, checks whether the two values are equal, and returns true if they are equal | (A = B) Return false. | |
Not equal, check whether the two values are equal, if they are not equal, return true | (A != B) Return true. | |
(A > B) Returns false. | ||
Greater than or equal sign, detects whether the value on the left is greater than or equal to the value on the right, and returns true if the value on the left is greater than or equal to the value on the right | (A >= B) returns false. | |
Less than or equal sign, detects whether the value on the left is less than or equal to the value on the right, and returns true if the value on the left is less than or equal to the value on the right | (A <= B) returns true. |
SELECT * from kxdang_tbl WHERE kxdang_author='菜鸟教程';
mysql> SELECT * from kxdang_tbl WHERE BINARY kxdang_author='kxdang.com'; Empty set (0.01 sec) mysql> SELECT * from kxdang_tbl WHERE BINARY kxdang_author='RUNOOB.COM'; +-----------+---------------+---------------+-----------------+ | kxdang_id | kxdang_title | kxdang_author | submission_date | +-----------+---------------+---------------+-----------------+ | 3 | JAVA 教程 | RUNOOB.COM | 2016-05-06 | | 4 | 学习 Python | RUNOOB.COM | 2016-03-06 | +-----------+---------------+---------------+-----------------+ 2 rows in set (0.01 sec)
BINARY keyword, which is case-sensitive, so kxdang_author The query condition of ='kxdang.com' has no data.
<?php $dbhost = 'localhost'; // 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"); // 读取 kxdang_author 为 RUNOOB.COM 的数据 $sql = 'SELECT kxdang_id, kxdang_title, kxdang_author, submission_date FROM kxdang_tbl WHERE kxdang_author="RUNOOB.COM"'; 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 WHERE 子句测试<h2>'; echo '<table border="1"><tr><td>教程 ID</td><td>标题</td><td>作者</td><td>提交日期</td></tr>'; while($row = mysqli_fetch_array($retval, MYSQLI_ASSOC)) { echo "<tr><td> {$row['kxdang_id']}</td> ". "<td>{$row['kxdang_title']} </td> ". "<td>{$row['kxdang_author']} </td> ". "<td>{$row['submission_date']} </td> ". "</tr>"; } echo '</table>'; // 释放内存 mysqli_free_result($retval); mysqli_close($conn); ?>
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