Common Table Expression (CTE) is a temporary result set that can be used within a SQL statement, typically a SELECT, INSERT, UPDATE or DELETE statement. CTEs are useful for creating self-contained queries that can be reused multiple times within a larger query or by other queries in a database. Here are some examples of SQL CTEs.
- Recursive CTE: A recursive Common Table Expression (CTE) is used to retrieve hierarchical data from a table. For example, if you have a table of employees with a manager field that points to the ID of another employee, you can use a recursive CTE to retrieve the entire hierarchical tree of employees. The recursive CTE builds on itself by repeatedly querying the same CTE until the desired result set is obtained.
- Pivot CTE: A pivot CTE is used to transform a row-based result set into a column-based result set. For example, if you have a table of sales data with a column for each month of the year, you can use a pivot CTE to transform the data into a result set with a row for each month and a column for each salesperson.
- Ranking CTE: A ranking CTE is used to assign a rank or row number to each row in a result set based on a specified order. For example, if you have a table of employees with a salary field, you can use a ranking CTE to assign a rank to each employee based on their salary.
- Filtering CTE: A filtering CTE is used to filter a result set based on a specific condition. For example, if you have a table of products with a column for the price of each product, you can use a filtering CTE to retrieve only the products with a price greater than a specified value.
- Inline CTE: An inline CTE is used to create a reusable subquery that can be referenced within a larger query. For example, if you have a complex query that involves multiple joins and subqueries, you can use an inline CTE to simplify the query by breaking it down into smaller, more manageable parts.
Common Table Expression (CTEs) are a powerful tool for creating complex queries that are easier to read and maintain. They can be used for a wide range of tasks, from retrieving hierarchical data to filtering and ranking result sets. By mastering the use of CTEs, SQL developers can create more efficient and effective queries that provide better insights into their data.
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