What is the method to remove duplicates from SQL query …

1 year ago
Sophia Anderson
2 minutes

There are several methods for removing duplicates from SQL query results.

  1. To eliminate duplicate rows from the query results, you can use the DISTINCT keyword in the SELECT statement. For example: SELECT DISTINCT column1, column2 FROM table_name;
  2. By using the GROUP BY clause, you can group the query results by one or more columns and then use aggregate functions (such as COUNT, SUM, etc.) to calculate the results for each group, achieving the effect of deduplication. For example: SELECT column1, column2 FROM table_name GROUP BY column1, column2;
  3. By using the HAVING clause: After the GROUP BY clause, filtering the grouped results can further eliminate duplicate rows by adding the HAVING clause. For example: SELECT column1, column2 FROM table_name GROUP BY column1, column2 HAVING COUNT(*) = 1;
  4. Using a subquery: By nesting a subquery within a query statement, you can use the DISTINCT keyword or GROUP BY clause in the subquery to remove duplicates, and then use the de-duplicated result as the main query’s result. For example: SELECT column1, column2 FROM (SELECT DISTINCT column1, column2 FROM table_name) subquery;
  5. By using window functions in databases that support them, you can use functions like ROW_NUMBER() and RANK() to group and sort query results, then select specific rows to achieve deduplication. For example: SELECT column1, column2 FROM (SELECT column1, column2, ROW_NUMBER() OVER (PARTITION BY column1, column2 ORDER BY column1) rn FROM table_name) subquery WHERE rn = 1;

Regardless of the method used, you can choose the most suitable way to deduplicate query results based on specific needs.

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