How to Select Cars That Passed All Tests from a Database Table?

Database Query: Identifying Cars Passing All Tests

This guide demonstrates how to efficiently select cars from a database that have successfully completed a predefined set of tests. We'll focus on a scenario where we need to identify cars in the passedtest table that have passed all tests (A, B, C, and D).

Limitations of the IN Statement

While the IN statement is useful for checking if a value exists within a list, it's unsuitable for this specific task. Using IN would incorrectly identify cars that passed even just one of the required tests. A more robust approach is needed.

Leveraging Aggregation and Grouping

The solution lies in using aggregate functions and grouping. We employ the COUNT() function to determine the number of distinct test types passed by each car. The GROUP BY clause then organizes the results by car name, allowing us to count distinct tests per car.

Filtering Results with HAVING

The HAVING clause is crucial for filtering the grouped results. It allows us to isolate cars that have a count of distinct test types equal to the total number of required tests (four, in this instance).

The SQL Solution

The following SQL query achieves the desired outcome:

<code class="language-sql">SELECT carname
FROM PassedTest
GROUP BY carname
HAVING COUNT(DISTINCT testtype) = 4;</code>

This query efficiently selects only the carnames from the passedtest table that have passed all four specified tests.

Expanding the Query for Comprehensive Data

To retrieve additional details about the cars that passed all tests, a nested query can be utilized:

<code class="language-sql">SELECT *
FROM cars
WHERE carname IN (
    SELECT carname
    FROM PassedTest
    GROUP BY carname
    HAVING COUNT(DISTINCT testtype) = 4
);</code>

This enhanced query retrieves all columns from the cars table for those cars identified as having passed all tests in the passedtest table.

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