sql: What does it mean

The meaning of COUNT(*) in SQL

In SQL, the COUNT(*) function is used to calculate the number of records in a table. It is an aggregate function that can be applied to any table or query regardless of its structure or content.

Note: COUNT(*) is different from COUNT(column), which only calculates the number of non-null values ​​of the specified column.

grammar

 <code>COUNT(*)</code>

result

This function returns the total number of records in the table, including null and duplicate values.

use

The COUNT(*) function is useful in the following cases:

• Find the number of records in the table
• Calculate a subset of records in a table
• Used as an aggregate function in grouping operations
• Used in subqueries to limit the number of results returned

Example

 <code>SELECT COUNT(*) FROM customers;</code>

This query returns the total number of records in the customers table.

 <code>SELECT COUNT(*) FROM orders WHERE order_date > '2023-01-01';</code>

This query will return the number of records in orders table with dates greater than 2023-01-01 .

 <code>SELECT department_id, COUNT(*) FROM employees GROUP BY department_id;</code>

This query returns a result set showing the number of employees in each department.

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