为什么这个 sql 片段总是返回 8 或 1？

Question

What is the result of:

WITH Tbl AS (SELECT 5 AS A UNION SELECT 6 AS A) 
SELECT COUNT(*) AS Tbl FROM Tbl AS A, Tbl AS B, Tbl AS C;

I know the result is supposed to be 8 but I don't know why. Also when I change both values (the 5 or 6) to the same thing it returns a table with the value 1 instead of 8 but all other instances it returns 8 no matter what numbers if they are different. I tested it out with an online sql executor.

Answer 1

Here is what the query does:

• the common table expression (the subquery within the with clause) generates a derived table made of two rows

• then, in the outer query, the from clause generates a cartesian product of this resultset twice: that's a total of 8 rows ( 2 * 2 * 2 )

• the select clause counts the number of rows - that's 8

The content of the rows in the with clause does not matter: this 5 and 6 could very well be foo and bar , or null and null , the result would be the same.

What makes a difference is the number of rows that the with clause generates. If it was generating just one row, you would get 1 as a result ( 1 * 1 * 1 ). If it was generating 3 rows, you would get 27 - and so on.

Answer 2

This expression:

WITH Tbl AS (SELECT 5 AS A UNION SELECT 6 AS A) 

creates a (derived) table with two rows.

This expression:

WITH Tbl AS (SELECT 5 AS A UNION SELECT 5 AS A) 

creates a (derived) table with one row, because UNION removes duplicates.

The rest of the query just counts the number of rows in the 3-way Cartesian product, which is either 1 1 1 or 2 2 2.