🔎 Значение WHERE в SQL: объяснение и примеры использования

What Does "WHERE" Mean in SQL?

SQL (Structured Query Language) is a powerful programming language used for managing and manipulating data in relational databases. One of the fundamental components of SQL is the "WHERE" clause, which allows you to filter and retrieve specific data from a table based on specified conditions. In this article, we will explore the meaning and usage of the "WHERE" clause in SQL.

Understanding the "WHERE" Clause

The "WHERE" clause is a conditional statement that is used to filter rows in a table based on certain criteria. It allows you to specify one or more conditions that must be met for a row to be included in the result set of a query. The conditions can be simple or complex, involving comparisons, logical operators, and even subqueries.

The basic syntax of the "WHERE" clause is as follows:

SELECT column1, column2, ...
FROM table_name
WHERE condition;

Here, "column1, column2, ..." represents the columns you want to retrieve from the table, "table_name" is the name of the table you want to query, and "condition" is the filtering criteria.

Using Operators in the "WHERE" Clause

The "WHERE" clause allows you to use various operators to compare values and define conditions. Here are some commonly used operators:

• Equal (=) Operator: The equal operator is used to check if a value is equal to another value. For example:

SELECT *
FROM customers
WHERE age = 25;

• Not Equal (<>) Operator: The not equal operator is used to check if a value is not equal to another value.

SELECT *
FROM customers
WHERE age <> 25;

• Greater Than (>) or Less Than (<) Operators: These operators are used to check if a value is greater than or less than another value.

SELECT *
FROM customers
WHERE age > 25;

• Logical Operators: SQL also supports logical operators such as "AND", "OR", and "NOT" for combining multiple conditions in the "WHERE" clause.

SELECT *
FROM customers
WHERE age > 25 AND city = 'New York';

Combining Conditions in the "WHERE" Clause

You can combine multiple conditions in the "WHERE" clause using logical operators to create more complex filtering criteria. For example, you can retrieve customers who are either from New York or have an age greater than 30:

SELECT *
FROM customers
WHERE age > 30 OR city = 'New York';

The possibilities for combining conditions are virtually limitless, allowing you to retrieve specific data based on your requirements.

Using the "WHERE" Clause with Other SQL Statements

The "WHERE" clause can be used with other SQL statements to perform various operations. Here are a few examples:

• SELECT Statement: The "WHERE" clause is commonly used with the "SELECT" statement to filter rows and retrieve specific data from a table.

SELECT *
FROM employees
WHERE salary > 50000;

• UPDATE Statement: The "WHERE" clause can be used with the "UPDATE" statement to modify specific rows in a table based on certain conditions.

UPDATE products
SET price = 10
WHERE category = 'Electronics';

• DELETE Statement: The "WHERE" clause can be used with the "DELETE" statement to remove specific rows from a table based on certain conditions.

DELETE FROM orders
WHERE status = 'cancelled';

Conclusion

The "WHERE" clause is a powerful tool in SQL that allows you to filter and retrieve specific data from a table based on specified conditions. By using comparison operators, logical operators, and combining multiple conditions, you can create complex filtering criteria to meet your specific requirements. Understanding the usage of the "WHERE" clause is essential for effectively querying and manipulating data in relational databases.