How to write query for duplicate data in sql

In SQL, two ways to query duplicate data are: use the GROUP BY clause to group the data and count the number of repetitions for each group. Use the EXCEPT clause to exclude a subquery that contains duplicate data from a subquery that contains all data.

Methods for querying duplicate data in SQL

In SQL, there are two methods for querying duplicate data:

1. GROUP BY clause

• Function: Group data and return the aggregated result (such as count) of each group.
• Syntax: SELECT column_list, COUNT(*) AS count FROM table_name GROUP BY column_list
• Usage: UseCOUNT(*) Aggregation function counts the number of repetitions for each group.

For example, query the product_id that appears repeatedly in the orders table:

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

2. EXCEPT clause

• Function: Exclude rows from one result set that are contained in another result set.
• Syntax: SELECT column_list FROM table_name EXCEPT SELECT column_list FROM table_name
• ##Usage: Convert a sub-section containing all data query as the first select list, and then use EXCEPT to include the subquery containing the duplicate data as the second select list.

product_id that appears only once in the orders table:

<code class="sql">SELECT product_id
FROM orders
EXCEPT
SELECT product_id
FROM orders
GROUP BY product_id
HAVING COUNT(*) > 1;</code>

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