![]() ![]() In the query above, we define two CTEs, london1_monthly_revenue and london2_monthly_revenue, to obtain the monthly revenue in 2021 for each branch in London. L1.revenue + l2.revenue AS london_revenue,įROM london1_monthly_revenue l1, london2_monthly_revenue l2 Here, we create two CTEs then join them in the main query. Suppose we want a report with the total monthly revenue in London in 2021, but we also want the revenue for each branch in London in the same report. By naming different parts of the query, CTEs make the query easy to read. In the next example, we show you how to divide and organize a long query using SQL CTEs. You can define two or more CTEs and use them in the main query. The report is ordered by max_daily_revenue in descending order.īelow are the results of this query. Then in the outer query, we obtain the highest amount of revenue for each branch in 2021. In this query, a CTE called daily_revenue has columns branch, date, and daily_revenue for each date in 2021. SUM(unit_price * quantity) AS daily_revenue In the next example, we generate a report with the highest daily revenue by branch. The result of the entire query is shown below: branch Finally, we join the result of the CTE highest with the sales table. Then, the outer query is executed, which uses the highest virtual table as if it were a regular table. The virtual table has columns branch, date, and highest_price, containing the branch name, the date, and the highest unit price sold that day in this branch, respectively. This query defines an SQL CTE called highest whose result creates a virtual table. To obtain the price of the most expensive item, we use a common table expression like this: In the first example, we obtain a report with the same records from the sales table but add an extra column with the price of the most expensive item sold in the same branch that day. The table sales, shown below, has one record per product sold: branch All examples are based on a database for a chain of cellular phone stores. In this section, we present some examples of SQL queries using common table expressions. Learning SQL Common Table Expressions by Example Other introductory articles include “ Improving Query Readability with Common Table Expressions” and “ When Should I Use a Common Table Expression (CTE)?” which explain common table expressions. ![]() There are many entry-level examples in this excellent introductory article on CTEs. ![]() Here, the main query (also known as the outer query) is SELECT a,c FROM my_cte WHERE …. The main query comes after the closing parenthesis and refers to the CTE. The CTE starts with the WITH keyword, after which you specify the name of your CTE, then the content of the query in parentheses. The name of this CTE is my_cte, and the CTE query is SELECT a,b,c FROM T1. Let's start with the syntax of a common table expression. It contains over 100 exercises that teach CTEs starting with the basics and progressing to advanced topics like recursive common table expressions. I recommend 's interactive Recursive Queries course. The best way to learn common table expressions is through practice. Each SQL CTE is like a named query, whose result is stored in a virtual table (a CTE) to be referenced later in the main query. CTEs often act as a bridge to transform the data in source tables to the format expected by the query.Ī common table expression, or CTE, is a temporary named result set created from a simple SELECT statement that can be used in a subsequent SELECT statement. CTEs work as virtual tables (with records and columns), created during the execution of a query, used by the query, and eliminated after query execution. The common table expression (CTE) is a powerful construct in SQL that helps simplify a query. ![]()
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