The role of where in sql

The WHERE clause is used to filter data by specifying conditions and retrieve only rows that meet the conditions. Detailed description: 1. Specify conditions and assign true values ​​to rows that meet the conditions; 2. Exclude other rows and the result only contains rows that meet the conditions; 3. For example, all customers from a specific state can be retrieved; 4. There are also sub- Other functions such as queries, aggregate functions, and joins.

The role of the WHERE clause

The WHERE clause is an important element in the SQL statement used to filter the data collection . It allows you to specify specific conditions and retrieve only rows that meet those conditions.

Detailed description of the function

The working principle of the WHERE clause is:

• Specify one or more conditions, which will A value of true is assigned to rows that meet the condition.
• After assigning a true value to a row that meets a condition, all other rows (i.e., false values) will be excluded from the query results.
• The result is a new data set containing only rows that meet the specified criteria.

Usage Example

For example, the following query will retrieve all customers from "California" from a table containing customer information:

<code class="sql">SELECT *
FROM customers
WHERE state = "CA";</code>

In this example, the WHERE clause is used to specify a condition, which is state = "CA". This will return all rows that meet this criteria, i.e. only customers from "California".

Other functions

In addition to filtering data, the WHERE clause has other functions:

• Subquery: The WHERE clause can be used to specify a subquery, that is, a query nested within another query.
• Aggregation functions: The WHERE clause can be used in conjunction with aggregate functions such as COUNT, SUM, and AVG to perform aggregation operations on data that meets specific conditions.
• Connection: The WHERE clause can be used to specify join conditions, which specifies how to establish a relationship between two tables.

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