In SQL, duplicate rows can occur when data is inserted without checking for existing duplicate values. This can lead to data integrity issues and make it difficult to work with the data. There are a few different ways to avoid duplicate rows in SQL, including using the UNIQUE constraint, the PRIMARY KEY constraint, or the ON DUPLICATE KEY UPDATE clause.
The UNIQUE constraint creates a unique index on a column or set of columns, which prevents duplicate values from being inserted. The PRIMARY KEY constraint creates a unique index on a column or set of columns and also identifies the row as the primary key of the table. The ON DUPLICATE KEY UPDATE clause allows you to specify an action to be taken when a duplicate value is inserted, such as updating the existing row.