How to query field duplicate data in SQL

Repeated field data can be queried using SQL's GROUP BY and COUNT() functions, DISTINCT and COUNT() functions, or window function ROW_NUMBER().

SQL query duplicate field data

Question: How to query duplicate field data using SQL?

answer:

In SQL, you can query duplicate field data using the following methods:

Method 1: Use the GROUP BY and COUNT() functions

<code class="sql">SELECT field_name, COUNT(*) AS count_duplicates FROM table_name GROUP BY field_name HAVING COUNT(*) > 1</code>

Example:

<code class="sql">SELECT customer_id, COUNT(*) AS count_duplicates FROM orders GROUP BY customer_id HAVING COUNT(*) > 1;</code>

Method 2: Use the DISTINCT and COUNT() functions

<code class="sql">SELECT field_name FROM table_name GROUP BY field_name HAVING COUNT(DISTINCT field_name) </code>

Example:

<code class="sql">SELECT customer_id FROM orders GROUP BY customer_id HAVING COUNT(DISTINCT customer_id) </code>

Method 3: Use window function ROW_NUMBER()

<code class="sql">SELECT field_name, ROW_NUMBER() OVER (PARTITION BY field_name ORDER BY field_name) AS row_number FROM table_name WHERE row_number > 1</code>

Example:

<code class="sql">SELECT customer_id, ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY customer_id) AS row_number FROM orders WHERE row_number > 1;</code>

explain:

The above method uses SQL functions and clauses to group data and calculate duplicate values. The GROUP BY clause groups data by field names, and the COUNT() function calculates the number of duplicates in each group. The HAVING clause filters out groups with duplicate counts greater than 1. The DISTINCT keyword ensures that only unique values ​​in each group are calculated. The ROW_NUMBER() window function assigns a sequence number to rows in each group and can be used to identify duplicate rows.

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