How <> works when compared with multiple values?

In this post, we’ll delve into the intricacies of how the <=> operator compares a single value to multiple values in Oracle SQL. We’ll explore an example query and dissect it to understand what happens behind the scenes.

Understanding the Problem

We have a table named MyTable with two columns: Col1 and Col2. The table has four rows of sample data:

CREATE TABLE MyTable(col1, col2) AS
SELECT 1, 'Val1' FROM DUAL UNION ALL
SELECT 2, 'Val2' FROM DUAL UNION ALL
SELECT 3, 'Val3' FROM DUAL UNION ALL
SELECT 4, 'Val4' FROM DUAL;

We have a query that uses the <=> operator to compare values:

SELECT *
FROM   MyTable A
WHERE  Col1 IN (2,3,4)
AND    NOT EXISTS(
         SELECT 1
         FROM MyTable B
         WHERE B.Col1 <> A.Col1
       );

Our goal is to understand how the <=> operator behaves when comparing a single value to multiple values in this query.

How <> Works

The <=> operator compares two values and returns one of three possible results:

• =: Equal
• <: Less than
• >: Greater than

In our example, we’re using the <=> operator with a single value (e.g., 2) compared to multiple values (2, 3, and 4).

To understand how this works, let’s examine the subquery that uses the NOT EXISTS clause:

SELECT *
FROM   MyTable B
WHERE  B.Col1 <> A.Col1;

For each row in the main query (A), we need to check if there exists a row in the subquery (B) where B.Col1 is not equal to A.Col1.

Step-by-Step Analysis

Let’s break down the analysis step by step:

1. For each value of Col1 in A, we’ll find all rows in B that satisfy the condition B.Col1 <> A.Col1.
2. If there is at least one row that satisfies this condition, then the NOT EXISTS clause will be false for that particular value of Col1.
3. We repeat this process for each value in the list (2, 3, and 4).

Understanding the Results

Let’s analyze the results:

• When Col1 is equal to 2:
  • There is one row in B that satisfies the condition (B.Col1 = 3). Therefore, NOT EXISTS is false.
  • The result for this value of Col1 will be included in the final output.
• When Col1 is equal to 3:
  • There are three rows in B that satisfy the condition (B.Col1 = 1, B.Col1 = 2, and B.Col1 = 4). Therefore, NOT EXISTS is false for all these values.
  • The result for this value of Col1 will be included in the final output (since there’s at least one row that satisfies the condition).
• When Col1 is equal to 4:
  • There are four rows in B that satisfy the condition (B.Col1 = 1, B.Col1 = 2, B.Col1 = 3, and B.Col1 = 4). Therefore, NOT EXISTS is false for all these values.
  • The result for this value of Col1 will be included in the final output (since there’s at least one row that satisfies the condition).

Conclusion

The <=> operator compares a single value to multiple values by checking if there exists at least one row where the two values are not equal. If such a row exists, then NOT EXISTS is false for all values in the list.

In our example query, since there exist rows in B that satisfy the condition for each value of Col1, the final output will be empty.

To illustrate this further, we can use an alternative approach using the EXISTS clause instead of NOT EXISTS. Here’s how:

SELECT *
FROM   MyTable A
WHERE  Col1 IN (2,3,4)
AND    EXISTS(
         SELECT 1
         FROM MyTable B
         WHERE B.Col1 <> A.Col1
       );

In this version, we’re using the EXISTS clause to check if there exists at least one row in B where B.Col1 is not equal to A.Col1.

The results will be identical: an empty output.

Alternative Approach

We can also rewrite the query using the IN operator instead of NOT EXISTS. Here’s how:

SELECT *
FROM   MyTable A
WHERE  Col1 IN (2,3,4)
AND    NOT (B.Col1 = A.Col1 OR B.Col1 = 1);

In this version, we’re using the IN operator to check if Col1 is equal to any of the values in the list. We’re then using a subquery with OR to check if there exists any row where B.Col1 is not equal to either A.Col1 or 1.

The results will be identical: an empty output.

Conclusion

In conclusion, the <=> operator compares a single value to multiple values by checking if there exists at least one row where the two values are not equal. When using this operator in conjunction with the NOT EXISTS clause, we need to ensure that there exist rows that satisfy the condition for each value in the list.

In our example query, since there exist rows in B that satisfy the condition for each value of Col1, the final output will be empty.

Last modified on 2024-05-20