What are the query statements in PHP?

There are three types of query statements in PHP: 1. SELECT statement, used to retrieve data from one or more data tables; 2. WHERE statement, used to filter the data in the data table specified in the FROM statement. Data under specific conditions; 3. ORDER BY statement, used to sort query results in a specific order (such as ascending or descending order).

Operating system for this tutorial: Windows 10 system, php8.1.3 version, Dell G3 computer.

The query statements in PHP are:

1. SELECT

The SELECT statement is used to retrieve data from one or more data tables.

The code is as follows:

$sql = "SELECT id, name, email FROM users";
$result = mysqli_query($conn, $sql);
// 循环遍历结果集
while ($row = mysqli_fetch_assoc($result)) {
    echo "ID: " . $row["id"] . " - Name: " . $row["name"] . " - Email: " . $row["email"] . "<br>";
}

This code selects the three fields id, name and email from the data table named "users" and stores the results in a file named in the variable $result. Subsequently, use the mysqli_fetch_assoc() function to obtain each row of data from the result set $result, and then output the corresponding id, name, and email values.

2. WHERE

The WHERE clause is used to filter data that meets specific conditions in the data table specified in the FROM statement.

$sql = "SELECT id, name, email FROM users WHERE age > 18";
$result = mysqli_query($conn, $sql);
// 循环遍历结果集
while ($row = mysqli_fetch_assoc($result)) {
    echo "ID: " . $row["id"] . " - Name: " . $row["name"] . " - Email: " . $row["email"] . "<br>";
}

This code selects the three fields id, name and email from the data table named "users" and adds a WHERE clause to filter age Users greater than 18. Subsequently, use the mysqli_fetch_assoc() function to obtain each row of data from the result set $result, and then output the corresponding id, name, and email values.

3. ORDER BY

The ORDER BY clause is used to sort query results in a specific order (such as ascending or descending order).

$sql = "SELECT name, age FROM users ORDER BY

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