Combining Results of Multiple SELECT Statements

In SQL, it’s common to need to combine the results of multiple SELECT statements into a single result set. This can be achieved using various techniques, including joining the results of subqueries, using conditional aggregation, or utilizing set operations like UNION. In this tutorial, we’ll explore these methods and provide examples to help you understand how to effectively combine the results of multiple SELECT statements.

Joining Results of Subqueries

One way to combine the results of multiple SELECT statements is by joining the results of subqueries. A subquery is a SELECT statement nested inside another SELECT statement. You can use subqueries in the FROM clause, and then join them using the same techniques you would use to join regular tables.

For example, suppose we have a table called Tasks with columns ks, Age, and Palt. We want to get the count of tasks for each ks value and the count of late tasks (where Age > Palt). We can achieve this by joining two subqueries:

SELECT 
    t1.ks, 
    t1.num_tasks AS "Total Tasks", 
    COALESCE(t2.late_tasks, 0) AS "Late Tasks"
FROM 
    (SELECT ks, COUNT(*) AS num_tasks FROM Tasks GROUP BY ks) t1
LEFT JOIN 
    (SELECT ks, COUNT(*) AS late_tasks FROM Tasks WHERE Age > Palt GROUP BY ks) t2
ON (t1.ks = t2.ks);

This query first calculates the total number of tasks for each ks value and then joins this result with another subquery that calculates the number of late tasks for each ks value. The LEFT JOIN ensures that all ks values are included, even if there are no late tasks.

Conditional Aggregation

Another approach to combining results is by using conditional aggregation. This involves using a single SELECT statement with aggregate functions like COUNT, SUM, or AVG, and applying conditions within these functions to calculate different values based on the condition.

Using the same example as above, we can rewrite the query to use conditional aggregation:

SELECT 
    ks,
    COUNT(*) AS "Total Tasks",
    SUM(CASE WHEN Age > Palt THEN 1 ELSE 0 END) AS "Late Tasks"
FROM 
    Tasks
GROUP BY 
    ks;

This query uses a single SELECT statement and applies the condition (Age > Palt) within the SUM function to count late tasks. This approach is more concise and efficient than joining subqueries.

Using UNION

The UNION operator is used to combine the result-set of two or more SELECT statements. Each SELECT statement within UNION must have the same number of columns, and the columns must have similar data types. Also, each SELECT statement within UNION should have a distinct set of rows.

However, using UNION to combine results in this scenario is not as straightforward because we need to align the columns (e.g., total tasks and late tasks) for each ks value. The previous methods provide more direct ways to achieve the desired outcome.

Choosing the Right Approach

• Joining Subqueries: Useful when you have complex calculations or when the subqueries are already defined and you just need to combine them.
• Conditional Aggregation: Often the most efficient method, especially for simple conditions. It minimizes the number of scans on the table and can be more readable.
• UNION: Best used when combining result sets from different tables or queries where the structure is identical but the data source differs.

In conclusion, combining the results of multiple SELECT statements in SQL can be achieved through various methods, each with its own advantages. The choice of method depends on the complexity of your queries, performance considerations, and personal preference. By mastering these techniques, you’ll be able to write more flexible and efficient SQL queries.