SQL: “no such column” with WITH Clause

Understanding Common Table Expressions (CTEs)

In recent years, the use of Common Table Expressions (CTEs) in SQL has become increasingly popular. A CTE is a temporary result set that is defined within the execution of a single statement. It allows you to perform complex queries and operations on data without having to rewrite your query every time.

Introduction to WITH Clause

The WITH clause is used to define a CTE. The basic syntax for a WITH clause is as follows:

WITH cte AS (
    SELECT column1, column2, ...
    FROM table_name
)
SELECT * FROM cte;

However, in the given Stack Overflow post, we can see that the query uses two separate SELECT statements. This is an alternative syntax for using a CTE.

The Problem with the Original Query

The original query provided by the Stack Overflow user contains two SELECT statements:

WITH avg_quantity AS (
    SELECT AVG(OrderDetails.Quantity) 
          FROM OrderDetails
)
SELECT Products.ProductName, OrderDetails.Quantity
FROM OrderDetails
JOIN Products ON Products.ProductID = OrderDetails.ProductID
WHERE OrderDetails.Quantity > avg_quantity;

The problem with this query is that the avg_quantity CTE is not being used correctly. The subquery SELECT AVG(OrderDetails.Quantity) FROM OrderDetails returns a scalar value, which cannot be directly compared to a table column.

The Correct Solution

To fix this issue, we need to use another SELECT statement to retrieve the average quantity from the CTE. This is where the alternative syntax for using a CTE comes into play:

WITH cte AS (
    SELECT AVG(Quantity) AS avg_quantity
    FROM OrderDetails
)
SELECT p.ProductName, od.Quantity
FROM OrderDetails od
INNER JOIN Products p ON p.ProductID = od.ProductID
WHERE od.Quantity > (SELECT avg_quantity FROM cte);

In this corrected query, we are using a subquery to retrieve the avg_quantity from the CTE.

How CTEs Behave Like Inlined SQL Code

Functionally speaking, a CTE behaves like inlined SQL code. It is not something like a user-defined variable which stores some value (although on some RDBMS, CTEs can be materialized).

When a query uses a CTE, the CTE is evaluated at runtime and its results are stored in a temporary result set. This temporary result set can then be joined with other tables using standard join operations.

A Deeper Look into How CTEs Work

To understand how CTEs work, let’s take a closer look at their internal workings.

Internal Working of CTEs

When a query uses a CTE, the database engine first evaluates the CTE. The result is stored in a temporary result set. This temporary result set can then be joined with other tables using standard join operations.

Here’s an example to illustrate this:

WITH cte AS (
    SELECT 1 AS value
)
SELECT * FROM cte;

In this query, the CTE cte contains a single row with a column named value. When we select from the CTE, the database engine evaluates the CTE and returns its result set.

However, if we try to use another SELECT statement to retrieve the value from the CTE, things get tricky:

WITH cte AS (
    SELECT 1 AS value
)
SELECT * FROM (SELECT value FROM cte);

In this query, we are trying to select the value column from the result set of the original CTE. However, since we are using a subquery to retrieve the value, the database engine does not evaluate the CTE again.

Instead, it evaluates the subquery and returns its result set. This is why we cannot directly compare the value to another column or use it in a conditional statement.

Temporal Behavior of CTEs

As mentioned earlier, on some RDBMS like Oracle, CTEs can be materialized, which means their results are stored in a table that can be queried like any other table.

However, this is not the case for most RDBMS. In general, CTEs behave like temporary result sets that are evaluated at runtime and discarded after the query completes.

Conclusion

In conclusion, understanding how CTEs work is essential to using them effectively in SQL queries. By recognizing their internal workings and temporal behavior, you can avoid common pitfalls and write more efficient queries.

Common Pitfalls to Avoid When Using CTEs

When working with CTEs, there are several pitfalls that you should avoid:

1. Missing SELECT Statement

One of the most common mistakes when using CTEs is forgetting to include a SELECT statement in the outer query.

Here’s an example:

WITH cte AS (
    SELECT AVG(Quantity) AS avg_quantity
    FROM OrderDetails
)
SELECT od.Quantity;

In this query, we are missing a SELECT statement in the outer query. This will result in an error because the database engine cannot determine what columns to select.

To fix this issue, you need to add a SELECT statement that includes all the desired columns:

WITH cte AS (
    SELECT AVG(Quantity) AS avg_quantity
    FROM OrderDetails
)
SELECT od.Quantity;

In this corrected query, we have added a SELECT statement that includes only the Quantity column.

2. Using Incorrect Data Types

Another common mistake when using CTEs is using incorrect data types in the outer query.

Here’s an example:

WITH cte AS (
    SELECT AVG(Quantity) AS avg_quantity
    FROM OrderDetails
)
SELECT od.Quantity;

In this query, we are selecting a Quantity column from the cte. However, if the database engine returns an integer value for the average quantity, this will result in an error.

To fix this issue, you need to use the correct data type for the outer query. For example:

WITH cte AS (
    SELECT AVG(Quantity) AS avg_quantity
    FROM OrderDetails
)
SELECT od.Quantity::integer;

In this corrected query, we have added a cast operator (::) to ensure that the Quantity column is returned as an integer value.

3. Not Joining with the CTE

Another common mistake when using CTEs is not joining with the CTE in the outer query.

Here’s an example:

WITH cte AS (
    SELECT AVG(Quantity) AS avg_quantity
    FROM OrderDetails
)
SELECT * FROM Products;

In this query, we are selecting all columns from the Products table without joining it with the CTE. This will result in an error because the database engine cannot determine how to join the tables.

To fix this issue, you need to add a join clause to join the Products table with the CTE:

WITH cte AS (
    SELECT AVG(Quantity) AS avg_quantity
    FROM OrderDetails
)
SELECT p.ProductName, od.Quantity
FROM Products p
INNER JOIN (SELECT Quantity FROM cte) od ON od.Quantity = p.ProductID;

In this corrected query, we have added a join clause to join the Products table with the CTE using an inner join.

Last modified on 2023-06-18