How to Combine Two SELECT Queries to Count Tasks and Late Tasks for Each Individual?

Combining SELECT Queries for Comprehensive Task Analysis

Database queries often require combining data from multiple sources for a complete picture. This involves joining the results of several SELECT statements—a crucial technique in many programming contexts.

Consider a scenario involving a task database with deadlines and individual assignments. The objective is to generate a report showing each individual, their total task count, and the number of overdue tasks.

This can be achieved using two separate SELECT queries: one to count tasks per individual, and another to count overdue tasks (where Age exceeds the predefined deadline PALT). To consolidate this information, a LEFT JOIN is employed. This ensures all individuals from the first query are included, with matching overdue task counts from the second query. The join is based on the ks column (individual identifier).

The combined query is as follows:

<code class="language-sql">SELECT t1.ks, t1.[# Tasks], COALESCE(t2.[# Late], 0) AS [# Late]
FROM 
    (SELECT ks, COUNT(*) AS '# Tasks' FROM Table GROUP BY ks) t1
LEFT JOIN
    (SELECT ks, COUNT(*) AS '# Late' FROM Table WHERE Age > Palt GROUP BY ks) t2
ON t1.ks = t2.ks;</code>

The output is a table listing each individual's ks, their total task count (# Tasks), and the number of overdue tasks (# Late). COALESCE handles cases where an individual has no overdue tasks, assigning a 0 value to # Late. This provides a comprehensive and accurate summary.

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