The role of where in mysql

The WHERE clause is used to filter MySQL query results based on conditions and include rows that meet the conditions in the results. It performs conditional filtering by specifying expressions, such as value comparison, range comparison, or logical operations, and supports complex queries and acquisition of specific data.

The role of the WHERE clause in MySQL

The WHERE clause is used in MySQL query statements to filter the returned results a powerful tool. It allows you to specify specific rows to be included in the query results.

Function

The main function of the WHERE clause is to filter data from the table based on specified conditions.

Syntax

<code>SELECT column_list
FROM table_name
WHERE condition;</code>

Where:

• condition is the expression used to filter rows.

• condition can be:

  • Value comparison (such as column_name = value)
  • Range comparison (Such as column_name BETWEEN value1 AND value2)
  • Logical operators (such as AND, OR, NOT)
  • Subquery

Usage

The WHERE clause is usually used for:

• Get specific data based on specific conditions (such as getting all orders with order number 1001)
• Exclude rows that do not meet the conditions (such as excluding canceled orders)
• Combine multiple conditions to further Refining the results (such as obtaining orders with order numbers greater than 1000 and order status "processed")

Example

<code>-- 获取所有名为 "John" 的客户
SELECT *
FROM customers
WHERE name = "John";</code>

<code>-- 获取订单总额大于 500 的所有订单
SELECT *
FROM orders
WHERE total_amount > 500;</code>

Notes

• The WHERE clause can only be used in SELECT statements.
• If you do not use the WHERE clause, the query will return all rows in the table.
• The condition in the WHERE clause must be a Boolean value (true or false).
• Conditions in the WHERE clause can reference columns in the table, functions, or subqueries.

The above is the detailed content of The role of where in mysql. For more information, please follow other related articles on the PHP Chinese website!