How to find duplicate records in a table on database - SQL tips

How to find duplicate records in table is a popular SQL interview question which has been asked as many times as difference between truncate and delete in SQL or finding second highest salary of employee. Both of these SQL queries are must know for any one who is appearing on any programming interview where some questions on database and SQL are expected. In order to find duplicate records in database table you need to confirm definition of duplicates, for example in below contact table which is suppose to store name and phone number of contact, a record is considered to be duplicate if both name and phone number is same but unique if either of them varies. Problem of duplicates in database arise when you don't have a primary key or unique key on database and that's why its recommended to have a key column in table. Anyway its easy to find duplicate records in table by using group by clause of ANSI SQL. Group by clause is used to group data based upon any column or a number of columns. Here in order to locate duplicate records we need to use group by clause on both name and phone as shown in second SQL SELECT query example. You can see in first query that it listed Ruby as duplicate record even though both Ruby have different phone number because we only performed group by on name. Once you have grouped data you can filter out duplicates by using having clause. Having clause is counter part of where clause for aggregation queries. Just remember to provide temporary name to count() data in order to use them in having clause.

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