How To Detect Duplicate Records Inwards A Tabular Array On Database - Sql Tips
How to detect duplicate records inwards tabular array is a pop SQL interview query which has been asked every bit many times every bit difference betwixt truncate together with delete inwards SQL or finding mo highest salary of employee. Both of these SQL queries are must know for whatever ane who is appearing on whatever programming interview where to a greater extent than or less questions on database together with SQL are expected. In lodge to find duplicate records inwards database table y'all demand to confirm Definition of duplicates, for instance inwards below contact tabular array which is suppose to shop name together with phone number of contact, a tape is considered to endure duplicate if both advert together with telephone expose is same but unique if either of them varies. Problem of duplicates inwards database arise when y'all don't receive got a primary fundamental or unique key on database together with that's why its recommended to receive got a fundamental column inwards table. Anyway its tardily to detect 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 expose of columns. Here inwards lodge to locate duplicate records nosotros demand to utilization group past times clause on both name together with phone every bit shown inwards second SQL SELECT query example. You tin compass the axe encounter inwards get-go query that it listed Ruby every bit duplicate tape fifty-fifty though both Ruby receive got dissimilar telephone expose because nosotros alone performed grouping past times on name. Once y'all receive got grouped information y'all tin compass the axe filter out duplicates past times using having clause. Having clause is counter purpose of where clause for aggregation queries. Just cry back to render temporary advert to count() information inwards lodge to utilization them inwards having clause.
SQL Query to detect duplicate records inwards a tabular array inwards MySQL
In this department nosotros volition encounter SQL query which tin compass the axe endure used to locate duplicate records inwards table. As explained inwards previous section, Definition of duplicate depends upon describe of piece of occupation concern rules which must endure used inwards grouping past times clause. In next query nosotros receive got used SELECT query to pick out all records from Contacts table. Here James, Johnny, Harry together with Ron are duplicated 4 times. mysql> pick out * 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 detect duplicates records based on name which mightiness non endure right if 2 contact of same but dissimilar numbers are stored, every bit inwards next lawsuit ready Ruby is shown every bit duplicate which is incorrect.
mysql> pick out 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 human face both advert together with telephone number together with alone impress duplicate if both advert together with telephone is same.
mysql> pick out 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 together with how many times they are duplicated inwards table.
mysql> pick out name, count(name) every bit times from contacts grouping past times name, telephone 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 detect duplicate records inwards table, These SQL queries volition operate on all database similar MySQL, Oracle, SQL Server together with Sybase every bit it alone uses ANSI SQL together with doesn't utilization whatever database specific feature. Another interesting SQL query interview query is "How to delete duplicate records from table" which nosotros volition encounter inwards to a greater extent than or less other post.
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