SQL: Selecting 1 ROW from a TABLE where 3 COLUMNS have repeating values

When working with relational databases, it’s common to encounter scenarios where you need to select data that appears in multiple rows due to repeated values. In this article, we’ll explore how to solve the problem of selecting only one row from a table where three columns have repeating values.

Understanding the Problem

Let’s consider an example to illustrate the issue at hand. We have a table table_a with the following structure:

We want to select only one row from table_a where we don’t care which values for COLUMN 2 (ID Numb) and COLUMN 3 (DESCR) we get. The resulting table should have a single row for each unique value in the COLUMN1 column.

Approach: Using Subqueries

One way to solve this problem is by using a subquery. Here’s an example of how you can achieve this:

SELECT 
    t1.COLUMN1, 
    t2.COLUMN2, 
    t3.COLUMN3
FROM (
    SELECT COLUMN1, ROW_NUMBER() OVER (PARTITION BY COLUMN1 ORDER BY ID Numb) AS row_num
    FROM table_a
) t1
LEFT JOIN table_b ON t1.COLUMN1 = table_b.NAME AND t1.row_num = 1
LEFT JOIN table_c ON t1.COLUMN1 = table_c.NAME AND t1.row_num = 1

In this query, we first use a subquery to assign a row number to each unique value in the COLUMN1 column. We then join this result with two other tables (table_b and table_c) on both the COLUMN1 column and the row number.

Approach: Using Window Functions

Another way to solve this problem is by using window functions, such as ROW_NUMBER(), RANK(), or DENSE_RANK().

Let’s consider an example using the ROW_NUMBER() function:

SELECT 
    COLUMN1, 
    COLUMN2, 
    COLUMN3
FROM (
    SELECT 
        COLUMN1, 
        COLUMN2, 
        COLUMN3,
        ROW_NUMBER() OVER (PARTITION BY COLUMN1 ORDER BY COLUMN2) AS row_num
    FROM table_a
) t
WHERE row_num = 1

In this query, we use the ROW_NUMBER() function to assign a unique number to each unique value in the COLUMN1 column. We then select only the rows with row_num = 1, which corresponds to the first occurrence of each unique value.

Approach: Using Common Table Expressions (CTEs)

We can also solve this problem using common table expressions (CTEs).

Let’s consider an example:

WITH cte AS (
    SELECT 
        COLUMN1, 
        COLUMN2, 
        COLUMN3,
        ROW_NUMBER() OVER (PARTITION BY COLUMN1 ORDER BY COLUMN2) AS row_num
    FROM table_a
)
SELECT * FROM cte WHERE row_num = 1

In this query, we define a CTE that assigns a unique number to each unique value in the COLUMN1 column. We then select only the rows with row_num = 1, which corresponds to the first occurrence of each unique value.

Conclusion

In conclusion, there are several approaches you can use to solve the problem of selecting one row from a table where three columns have repeating values. By using subqueries, window functions, or common table expressions (CTEs), you can select only the rows that meet your specific requirements.

When choosing an approach, consider the following factors:

• Complexity: How complex is your query? If it’s relatively simple, a CTE might be sufficient. However, if it’s more complicated, a subquery or window function might be better.
• Performance: Which approach performs better in terms of speed and efficiency?
• Readability: How readable is the code? Choose an approach that makes your code easy to understand for you and others.

By understanding these approaches and choosing the right one for your use case, you can effectively solve problems involving repeated values in SQL.

Last modified on 2025-02-17