SQL Serve Common Table Expressions (CTEs)

 

   A common table expression (CTE) is a temporary result set that you can reference within a SELECT, INSERT, UPDATE, DELETE, or CREATE VIEW statement. CTEs are a powerful tool in SQL Server, as they allow you to define a reusable set of rows within a query, and can be used to simplify complex queries by breaking them up into smaller, more manageable pieces.

Here's an example of how to use a CTE in a SELECT statement:

WITH cte AS (

  SELECT col1, col2 FROM my_table WHERE col3 = 'value'

)

SELECT * FROM cte WHERE col1 > 10

    In this example, the CTE is defined as a SELECT statement that retrieves rows from the my_table table where col3 equals 'value'. The CTE is then referenced in the outer SELECT statement, which retrieves all rows from the CTE where col1 is greater than 10.

    CTEs can also be used in other types of statements, such as INSERT, UPDATE, DELETE, and CREATE VIEW. For example:

WITH cte AS (

  SELECT col1, col2 FROM my_table WHERE col3 = 'value'

)

INSERT INTO my_other_table (col1, col2)

SELECT col1, col2 FROM cte WHERE col1 > 10

   In this example, the CTE is used in an INSERT statement to insert rows into the my_other_table table.

   CTEs are a useful tool for simplifying and organizing complex queries in SQL Server. By breaking a query up into smaller, more manageable pieces, you can make your code easier to read and maintain.

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