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.

For example, query the

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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