Using DISTINCT in SQL Queries to Select Columns from Multiple Tables

When working with multiple tables and trying to retrieve data based on specific conditions, you often need to use SELECT statements along with various techniques to filter the results. One common technique is using the DISTINCT keyword to select unique values from a table or column.

Understanding the Problem Statement

The given problem involves a SQL query that joins three tables: TABLE_A, TABLE_B, and TABLE_C. The goal is to retrieve specific columns from these tables based on certain conditions. Specifically, it requires selecting all columns (some_id) from TABLE_A where:

If key_1 exists in TABLE_B and its value equals the corresponding value in TABLE_B.key_1, then select that row.
Otherwise, if key_2 exists in TABLE_B and its value equals the corresponding value in TABLE_B.key_2, then select that row.

However, there’s an additional requirement: if neither key_1 nor key_2 exists, select rows from TABLE_A where key_2 is present in TABLE_B.

Solution Overview

To solve this problem efficiently and correctly, we can follow these steps:

Join the required tables (TABLE_A, TABLE_B, and TABLE_C) using their respective columns.
Apply filters to limit the number of rows that match our requirements.

We will leverage SQL’s powerful filtering capabilities by utilizing the following techniques:

Filtering based on existing table data
Using aggregate functions like COALESCE or IS NOT NULL
Utilizing SQL joins and conditions

Implementing the Solution in SQL

Below is an example of how you can implement this solution using standard SQL practices.

SELECT tbl_a.some_id 
FROM TABLE_B tbl_b JOIN TABLE_A tbl_a
ON tbl_a.tableB_id = tbl_b.some_id
JOIN   TABLE_C tbl_c
ON tbl_b.tableC_id = tbl_c.some_id 
WHERE tbl_a.entity_id = 8 AND
      (tbl_b.isSomeCheck = 1 OR tbl_b.isOtherCheck = 1) AND
      COALESCE(tbl_b.key_1, tbl_b.key2) IS NOT NULL
ORDER BY tbl_a.position;

This query meets the requirements stated in your problem by:

Filtering for rows where tbl_a.entity_id matches with a certain value (in this case, 8)
Ensuring that either key_1 or key_2 exists from TABLE_B before considering rows from TABLE_A
Using COALESCE to check if any of the values in KEY_1 and KEY_2 are present in TABLE_B

The result is a filtered list of columns (some_id) from TABLE_A, ordered by their position.

Additional Advice

You can further improve your SQL query performance by applying indexes on relevant columns (e.g., table IDs, keys) to increase data retrieval speed.

For production environments, always test and validate your SQL queries thoroughly before deploying them in live situations.

Last modified on 2025-02-02