How to filter duplicates in sql

Methods for filtering duplicate records in SQL include: using the DISTINCT keyword to remove duplicate values. Use the GROUP BY clause to group the results and return the first row of each group. Use the COUNT() function to find the values ​​that appear multiple times and filter them further. Use a temporary table to save unique values ​​and retrieve records from them. Use the window function to assign a unique line number to each line and select the first non-repeat line.

Methods for filtering duplicate records in SQL

There are several ways to filter duplicate records in SQL, depending on the database system and the SQL dialect used.

Using DISTINCT Key

 <code class="sql">SELECT DISTINCT column_name FROM table_name;</code>

Use the DISTINCT keyword to remove duplicate values ​​from the result set. It returns only the unique value of each row in the result set.

Use the GROUP BY clause

 <code class="sql">SELECT column_name FROM table_name GROUP BY column_name;</code>

Use the GROUP BY clause to group the result sets and return the first row of each group as a result. This removes duplicate values, but may not work in all cases.

Use the COUNT() function

 <code class="sql">SELECT column_name FROM table_name WHERE COUNT(*) > 1;</code>

Use the COUNT() function in conjunction with the WHERE clause to find values ​​that occur more than once. You can then use this result set to filter the table further.

Use temporary tables

 <code class="sql">CREATE TEMP TABLE unique_table AS SELECT DISTINCT column_name FROM table_name; SELECT * FROM unique_table;</code>

Use a temporary table to save unique values ​​of a result set. You can then run a query against the temporary table to retrieve unique records.

Using window functions

 <code class="sql">SELECT column_name, ROW_NUMBER() OVER (PARTITION BY column_name ORDER BY column_name) AS row_num FROM table_name; SELECT column_name FROM ( SELECT column_name, ROW_NUMBER() OVER (PARTITION BY column_name ORDER BY column_name) AS row_num FROM table_name ) AS subquery WHERE row_num = 1;</code>

Use window functions to assign a unique row number to a specific column of each row. You can then use this line number to select only the first line (no duplicate).

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