Command to query duplicate data in sql

The way to query duplicate data in SQL is to use the GROUP BY clause to group the required columns. Use the HAVING clause to specify filter conditions for duplicate data.

SQL command to query duplicate data

How to query duplicate data

In SQL, querying duplicate data can be achieved by using the GROUP BY and HAVING clauses.

Steps

1. Group the desired columns using the GROUP BY clause.
2. Use the HAVING clause to specify filter conditions for duplicate data.

Syntax

<code class="sql">SELECT column_name(s)
FROM table_name
GROUP BY column_name(s)
HAVING COUNT(*) > 1;</code>

Example

Suppose there is a table named students , containing the following data:

student_id	name
1	John Doe
2	Jane Smith
3	John Doe
4	Mary Johnson

To query the duplicate name in the students table fields, you can use the following query:

<code class="sql">SELECT name
FROM students
GROUP BY name
HAVING COUNT(*) > 1;</code>

Output

<code>John Doe</code>

Other examples

• Queryorders Duplicate product_id fields in the table:

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

• Queryemployees Duplicate email## in the table # field and display the number of repetitions:

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

Note

##COUNT(*)
• The function counts the number of repetitions in each group Rows. The conditions in the
HAVING
• clause can be based on aggregate functions such as COUNT, SUM, and AVG.

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