In SQL, how can you show only one row for each duplicate key value in a table?

 In SQL, to show only one row for each duplicate key value in a table, you can use different methods depending on the database system you're using and the specific needs of your query. Below are some common approaches to achieve this:

1. Using DISTINCT

If you want to select unique rows based on the entire row content (i.e., no two rows with the same values in all columns), you can use DISTINCT.

Example:

SELECT DISTINCT column1, column2, column3
FROM your_table;

2. Using GROUP BY

If you're looking to group rows by a particular column (the duplicate key column) and want to retrieve only one row per duplicate key, you can use GROUP BY. You can use aggregate functions like MIN(), MAX(), or others if necessary for other columns.

Example:

SELECT column1, MAX(column2) AS column2
FROM your_table
GROUP BY column1;

This query returns one row for each column1 value and shows the maximum value of column2 for each group.

3. Using ROW_NUMBER() Window Function (for more control)

For more control, especially when you want to retrieve the first occurrence of each duplicate key value while retaining all columns, you can use the ROW_NUMBER() window function. This is useful if you want to filter rows based on specific criteria, such as keeping the "first" row based on a certain column.

Example:

WITH RankedRows AS (
    SELECT *,
           ROW_NUMBER() OVER (PARTITION BY column1 ORDER BY column2) AS rn
    FROM your_table
)
SELECT *
FROM RankedRows
WHERE rn = 1;

This query assigns a unique row number to each row partitioned by column1. The ORDER BY clause determines which row will be considered the "first" (you can adjust the ordering criteria based on your needs). Then, it filters to return only the first occurrence (rn = 1) of each duplicate key.

4. Using DISTINCT ON (PostgreSQL-specific)

If you're using PostgreSQL, you can use DISTINCT ON to select one row for each duplicate key value.

Example:

SELECT DISTINCT ON (column1) column1, column2
FROM your_table
ORDER BY column1, column2;

This query selects the first row for each column1 based on the ordering of column2.

Conclusion

• Use DISTINCT for simple deduplication of rows.
• Use GROUP BY when you need to aggregate data or just select unique keys.
• Use ROW_NUMBER() for more complex scenarios where you need to filter rows and keep the first or a specific one.
• For PostgreSQL, DISTINCT ON is a powerful option for getting the first row per key value.