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Judul : How To Notice Duplicate Records Inward A Tabular Array On Database - Sql Tips
link : How To Notice Duplicate Records Inward A Tabular Array On Database - Sql Tips

How To Notice Duplicate Records Inward A Tabular Array On Database - Sql Tips

How to notice duplicate records inwards tabular array is a pop SQL interview query which has been asked equally many times equally difference betwixt truncate as well as delete inwards SQL or finding minute highest salary of employee. Both of these SQL queries are must know for whatever ane who is appearing on whatever programming interview where around questions on database as well as SQL are expected. In club to find duplicate records inwards database table you lot demand to confirm Definition of duplicates, for instance inwards below contact tabular array which is suppose to shop name as well as phone number of contact, a tape is considered to last duplicate if both advert as well as holler release is same but unique if either of them varies. Problem of duplicates inwards database arise when you lot don't guide keep a primary substitution or unique key on database as well as that's why its recommended to guide keep a substitution column inwards table. Anyway its tardily to notice duplicate records inwards tabular array past times using group past times clause of ANSI SQL. Group past times clause is used to grouping information based upon whatever column or a release of columns. Here inwards club to locate duplicate records nosotros demand to  exercise group past times clause on both name as well as phone equally shown inwards second SQL SELECT query example. You tin run across inwards starting fourth dimension query that it listed Ruby equally duplicate tape fifty-fifty though both Ruby guide keep dissimilar holler release because nosotros alone performed grouping past times on name. Once you lot guide keep grouped information you lot tin filter out duplicates past times using having clause. Having clause is counter purpose of where clause for aggregation queries. Just call back to render temporary advert to count() information inwards club to exercise them inwards having clause.

SQL Query to notice duplicate records inwards a tabular array inwards MySQL

In this department nosotros volition run across SQL query which tin last used to locate duplicate records inwards table. As explained inwards previous section, Definition of duplicate depends upon draw of piece of job organization rules which must last used inwards grouping past times clause. In next query nosotros guide keep used SELECT query to guide all records from Contacts table. Here James, Johnny, Harry as well as Ron are duplicated iv times.

mysql> guide * from Contacts;

+-------+----------+

| name  | phone    |

+-------+----------+

| James | 80983243 |

| Johnny | 67543212 |

| Harry | 12341234 |

| Ron   | 44446666 |

| James | 80983243 |

| Johnny | 67543212 |

| Harry | 12341234 |

| Ron   | 44446666 |

| James | 80983243 |

| Johnny | 67543212 |

| Harry | 12341234 |

| Ron   | 44446666 |

| James | 80983243 |

| Johnny | 67543212 |

| Harry | 12341234 |

| Ron   | 44446666 |

| Ruby  |  8965342 |

| Ruby  |  6888342 |

+-------+----------+

18 rows inwards ready (0.00 sec)

Following SELECT query volition only notice duplicates records based on name which mightiness non last right if 2 contact of same but dissimilar numbers are stored, equally inwards next lawsuit ready Ruby is shown equally duplicate which is incorrect.

mysql> guide name, count(name) from contacts grouping past times name;

+-------+-------------+

| name  | count(name) |

+-------+-------------+

| Harry |           4 |

| James |           4 |

| Johnny |           4 |

| Ron   |           4 |

| Ruby  |           2 |

+-------+-------------+

5 rows inwards ready (0.00 sec)

This is the right agency of finding duplicate contacts at it facial expression both advert as well as holler number as well as alone impress duplicate if both advert as well as holler is same.

mysql> guide name, count(name) from contacts grouping past times name, phone;

+-------+-------------+

| name  | count(name) |

+-------+-------------+

| Harry |           4 |

| James |           4 |

| Johnny |           4 |

| Ron   |           4 |

| Ruby  |           1 |

| Ruby  |           1 |

+-------+-------------+

6 rows inwards ready (0.00 sec)

having clause inwards SQL query volition filter duplicate records from non duplicate records. As inwards next query it impress all duplicate records as well as how many times they are duplicated inwards table.

mysql> guide name, count(name) equally times from contacts grouping past times name, holler having times>1;

+-------+-------+

| name  | times |

+-------+-------+

| Harry |     4 |

| James |     4 |

| Johnny |     4 |

| Ron   |     4 |

+-------+-------+

4 rows inwards ready (0.00 sec)

That's all on how to notice duplicate records inwards table, These SQL queries volition guide house all database similar MySQL, Oracle, SQL Server as well as Sybase equally it alone uses ANSI SQL as well as doesn't exercise whatever database specific feature. Another interesting SQL query interview query is "How to delete duplicate records from table" which nosotros volition run across inwards around other post.

Further Learning
What is ACID properties of database transaction

Difference betwixt Correlated as well as Noncorrelated subquery inwards SQL

Difference betwixt clustered as well as nonclustered index inwards database

Difference betwixt principal substitution as well as candidate substitution inwards SQL

How to guide keep backup of MySQL database using mysqldump utility

10 ofttimes used MySQL ascendency for beginners

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