A Common Table Expression (CTE) provides a way to create temporary result sets that can be referenced within a SELECT
, INSERT
, UPDATE
, or DELETE
statement. CTEs can make your SQL queries more readable and modular. They are particularly useful in simplifying complex queries, such as those involving recursive operations or multiple levels of aggregation.
The basic syntax of a CTE is:
WITH CTE_Name AS (
SELECT column1, column2, ...
FROM table_name
WHERE condition
-- Other clauses like GROUP BY, HAVING, etc.
)
SELECT * FROM CTE_Name;
Here, CTE_Name
is the temporary name given to the result set produced by the CTE. This name is then used in the main SELECT
query to refer to the result set. The CTE is defined by a SELECT
statement that can include various clauses such as WHERE
, GROUP BY
, and HAVING
to filter and summarize the data.
CTEs are ideal for temporary result sets where you need to break down complex logic into simpler parts. They are widely used for recursive operations, data hierarchies, running totals, and when performing row-by-row operations that would otherwise require cumbersome subqueries or multiple queries.
In summary, CTEs enhance the power and flexibility of SQL, allowing for more structured, readable, and efficient query design. They are a fundamental tool for anyone looking to perform advanced data manipulation and analysis within relational databases.