mysql中使用where限定查询条件

Summary: you will learn how to use MySQL WHERE clause to filter rows returned from the SELECT statement.

If you use the SELECT statement to query the data from a table without the WHERE clause, you will get all rows in the  table, which sometimes brings more data than you need. The WHERE clause allows you to specify exact rows to select based on given conditions e.g., find all customers in the U.S.

The following query selects all customers whose country is U.S. from the customers table. We use the WHEREclause to filter the customers. In the WHERE clause, we compare the values of the country column with the USA literal string.

SELECT customerName, city FROM customers WHERE country = 'USA';

You can form a simple condition like the query above, or a very complex one that combines multiple expressions with logical operators such as AND and OR. For example, to find all customers in the U.S . and also in the New York city, you use the following query:

SELECT customerName, city FROM customers WHERE country = 'USA' AND        city    = 'NYC';

You can test the condition for not only equality but also inequality. For example, to find all customers whose credit limit is greater than 200.000 USD, you use the following query:

SELECT customerName, creditlimit FROM customers WHERE creditlimit > 200000;

There are several useful operators that you can use in the WHERE clause to form more practical queries such as:

• BETWEEN selects values within a range of values.

• LIKE matches value based on pattern matching.

• IN specifies if the value matches any value in a list.

• IS NULL checks if the value is NULL

The WHERE clause is used not only with the SELECT statement but also other SQL statements to filter rows such as DELETE and UPDATE.

In this tutorial, we’ve shown you how to use MySQL WHERE clause to filter records based on conditions.

Related Tutorials

• Updating Data Using MySQL UPDATE Statement

原文链接：http://outofmemory.cn/mysql/mysql-where