mysql query duplicate records

MySQL is a widely used relational database management system with powerful data query and management capabilities. When using MySQL, you often encounter situations where you need to query duplicate records. This article will introduce how to query for duplicate records in MySQL.

1. Use the GROUP BY statement to query duplicate records

In MySQL, you can use the GROUP BY statement to query duplicate records. The GROUP BY statement groups identical data and performs calculations on each group. By filtering the calculation results, duplicate records can be filtered out.

For example, if you want to query for duplicate names and email addresses in the table, you can use the following SQL statement:

SELECT name, email, COUNT(*) 
FROM users 
GROUP BY name, email 
HAVING COUNT(*) > 1;

This statement uses the GROUP BY statement to group the same names and email addresses. and count each group. If the count result is greater than 1, it is a duplicate record. The final query results include name, email address and count value.

2. Use the DISTINCT keyword to query duplicate records

In addition to using the GROUP BY statement, you can also use the DISTINCT keyword to query duplicate records. The DISTINCT keyword is used to remove duplicate records.

For example, if you want to query for duplicate email addresses in the table, you can use the following SQL statement:

SELECT DISTINCT email 
FROM users 
GROUP BY email 
HAVING COUNT(*) > 1;

This statement uses the DISTINCT keyword to remove duplicate email addresses, and then uses the GROUP BY statement Group identical email addresses and count each group. If the count result is greater than 1, it is a duplicate email address. The final query results only include duplicate email addresses.

3. Use the SELF JOIN statement to query duplicate records

In addition to using the GROUP BY statement and the DISTINCT keyword, you can also use the SELF JOIN statement to query duplicate records. The SELF JOIN statement is used to join records in the same table.

For example, if you want to query for duplicate email addresses in the table, you can use the following SQL statement:

SELECT u1.email 
FROM users u1 
JOIN users u2 
ON u1.email = u2.email AND u1.id <> u2.id;

This statement uses the SELF JOIN statement to join records with the same email address in the same table. Together. By satisfying the conditions of u1.email = u2.email and u1.id <> u2.id at the same time, duplicate records can be found. The final query results only include duplicate email addresses.

The above are several methods for querying duplicate records in MySQL. Each method has its applicable situations. Just choose the appropriate method for query according to actual needs.

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