MySQL WHERE child clause
We know to use SQL SELECT statement to read data from MySQL table.
To conditionally select data from a table, add a WHERE clause to the SELECT statement.
grammar
Following is the general syntax of SQL SELECT statement to read data from the data table using WHERE clause:
SELECT field1, field2,...fieldN FROM table_name1, table_name2...
[WHERE condition1 [AND [OR]] condition2.....
- In the query statement, you can use one or more tables, use commas to separate the tables, and use the WHERE statement to set the query conditions.
- You can specify any conditions in the WHERE clause.
- You can specify one or more conditions using AND or OR.
- The WHERE clause can also be used in SQL DELETE or UPDATE commands.
- The WHERE clause is similar to the if condition in the programming language, and reads the specified data according to the field value in the MySQL table.
The following is a list of operators that can be used in the WHERE clause.
The examples in the table below assume that A is 10 and B is 20
| operator | describe | example |
|---|---|---|
| = | Equal sign, checks whether two values are equal, and returns true if they are equal | (A = B) returns false. |
| <>, != | Not equal, checks whether two values are equal, returns true if not equal | (A != B) returns true. |
| > | Greater than sign, check whether the value on the left is greater than the value on the right, and return true if the value on the left is greater than the value on the right | (A > B) returns false. |
| < | Less than sign, check whether the value on the left is less than the value on the right, and return true if the value on the left is less than the value on the right | (A < B) returns true. |
| >= | Greater than or equal sign, check whether the value on the left is greater than or equal to the value on the right, and return 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, check whether the value on the left is less than or equal to the value on the right, and return true if the value on the left is less than or equal to the value on the right | (A <= B) returns true. |
WHERE clause is very useful if we want to read specified data in MySQL data table.
Using the primary key as a conditional query in the WHERE clause is very fast.
If the given condition does not have any matching records in the table, then the query will not return any data.
Read data from command prompt
We will use the WHERE clause in the SQL SELECT statement to read the data in the MySQL data table kxdang_tbl:
example
The following example will read all records in the kxdang_tbl table whose kxdang_author field value is Sanjay:
SQL SELECT WHERE child clause
SELECT * from kxdang_tbl WHERE kxdang_author='菜鸟教程';
Output result:
String comparisons for MySQL's WHERE clause are case-insensitive. You can use the BINARY keyword to set the string comparison of the WHERE clause to be case-sensitive.
Examples are as follows:
BINARY keyword
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)
The BINARY keyword is used in the example , which is case-sensitive, so the query condition of kxdang_author='kxdang.com' has no data.
Read data using PHP script
You can use the PHP function mysqli_query() and the same SQL SELECT command with a WHERE clause to get data.
This function is used to execute the SQL command, and then output all the query data through the PHP function mysqli_fetch_array().
example
The following example will return records from the kxdang_tbl table that use the kxdang_author field value as RUNOOB.COM:
MySQL WHERE clause test:
<?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);
?>