sql - How to get only one row against each id in MySql

Sql - How to get only one row against each id in MySql

If you want to retrieve only one row per unique ID in MySQL, you can use the DISTINCT keyword along with the GROUP BY clause. Here's an example:

Assuming you have a table named your_table with a column named id and other columns, and you want to retrieve one row for each unique ID:

SELECT id, MAX(column1) AS column1, -- Replace with your column names MAX(column2) AS column2, MAX(column3) AS column3 FROM your_table GROUP BY id;

Replace your_table, id, and the column names (column1, column2, column3, etc.) with your actual table name, ID column name, and other column names.

Explanation:

  • The GROUP BY id groups the rows based on the unique ID values.
  • The aggregate functions (e.g., MAX) are used to select one value for each column within each group.

This example uses MAX as an aggregate function, but you can use other aggregate functions based on your requirements (e.g., MIN, SUM, etc.). The goal is to aggregate the values for each column within each group.

Adjust the query according to your specific table structure and the columns you want to include in the result.

Examples

  1. ROW_NUMBER() Window Function:

    • SELECT id, column1, column2 FROM ( SELECT id, column1, column2, ROW_NUMBER() OVER (PARTITION BY id ORDER BY some_column) as row_num FROM your_table ) ranked WHERE row_num = 1;
    • Description: Assigns a row number to each row within a partition of the same id and selects the rows where the row number is 1.

  2. Using INNER JOIN with Subquery:

    • SELECT t1.* FROM your_table t1 JOIN ( SELECT id, MAX(some_column) as max_column FROM your_table GROUP BY id ) t2 ON t1.id = t2.id AND t1.some_column = t2.max_column;
    • Description: Joins the table with a subquery that finds the maximum value for a specific column grouped by id.

  3. Self-Join with Subquery:

    • SELECT t1.* FROM your_table t1 LEFT JOIN your_table t2 ON t1.id = t2.id AND t1.timestamp < t2.timestamp WHERE t2.id IS NULL;
    • Description: Performs a self-join to find rows where there is no other row with a higher timestamp for the same id.

  4. Subquery with NOT EXISTS:

    • SELECT t1.* FROM your_table t1 WHERE NOT EXISTS ( SELECT 1 FROM your_table t2 WHERE t2.id = t1.id AND t2.timestamp > t1.timestamp );
    • Description: Selects rows where there is no other row with a higher timestamp for the same id.

  5. Using DISTINCT and INNER JOIN:

    • SELECT t1.* FROM your_table t1 INNER JOIN ( SELECT id, MIN(timestamp) as min_timestamp FROM your_table GROUP BY id ) t2 ON t1.id = t2.id AND t1.timestamp = t2.min_timestamp;
    • Description: Uses a combination of DISTINCT and INNER JOIN to select rows with the minimum timestamp for each id.

  6. Subquery with LIMIT 1:

    • SELECT id, column1, column2 FROM your_table t1 WHERE timestamp = (SELECT MAX(timestamp) FROM your_table t2 WHERE t2.id = t1.id LIMIT 1);
    • Description: Uses a subquery with LIMIT 1 to select rows with the maximum timestamp for each id.

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