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How to Find Duplicates Values Across Multiple Columns in SQL?

  • Difficulty Level : Expert
  • Last Updated : 19 Nov, 2021

In SQL, sometimes we need to find duplicate entries across multiple columns in a table in a single query. We will use the GROUP BY and COUNT keywords to achieve this. For this, we use a specific kind of query shown in the below demonstration. For this article, we will be using the Microsoft SQL Server as our database and Select keyword.

Step 1: Create a Database. For this use the below command to create a database named GeeksForGeeks.

Query:

CREATE DATABASE GeeksForGeeks

Output:

Step 2: Use the GeeksForGeeks database. For this use the below command.

Query:

USE GeeksForGeeks

Output:

Step 3: Create a table of POSTINGS inside the database GeeksForGeeks. This table has 4 columns namely POSTING_ID, OFFICER_NAME, TEAM_SIZE and POSTING_LOCATION containing the id, the name of the officer, the size of the team and locations of the postings of Foreign Services in different countries.

Query:

CREATE TABLE POSTINGS(
POSTING_ID INT,
OFFICER_NAME VARCHAR(10),
TEAM_SIZE INT,
POSTING_LOCATION VARCHAR(10));

Output:

Step 4: Describe the structure of the table POSTINGS.

Query:

EXEC SP_COLUMNS POSTINGS;

Output:

Step 5: Insert 5 rows into the POSTINGS table.

Query:

INSERT INTO POSTINGS VALUES(1,'RYAN',10,'GERMANY');
INSERT INTO POSTINGS VALUES(2,'JACK',6,'ROMANIA');
INSERT INTO POSTINGS VALUES(3,'JANE',4,'HAWAII');
INSERT INTO POSTINGS VALUES(4,'JIM',10,'GERMANY');
INSERT INTO POSTINGS VALUES(5,'TIM',10,'GERMANY');
INSERT INTO POSTINGS VALUES(6,'RYAN',11,'GERMANY');
INSERT INTO POSTINGS VALUES(7,'RYAN',10,'GERMANY');
INSERT INTO POSTINGS VALUES(8,'RYAN',10,'GERMANY');
INSERT INTO POSTINGS VALUES(9,'JACK',6,'CUBA');
INSERT INTO POSTINGS VALUES(10,'JACK',6,'HAITI');

Output:

Step 6: Display all the rows of the POSTINGS table.

Query:

SELECT * FROM POSTINGS;

Output:

Step 7: Find duplicates in 3(multiple) columns i.e. in OFFICER_NAME, TEAM_SIZE and POSTING_LOCATION in the table POSTINGS. To achieve the, we need to group the records by these three columns and display those which have the count greater than 1 i.e. have matching values. Use the keywords GROUP BY and COUNT.

Syntax:

SELECT COLUMN_NAME1, COLUMN_NAME2, 
COLUMN_NAME3, COUNT(*) AS ALIAS
FROM TABLE_NAME
GROUP BY COLUMN_NAME1, COLUMN_NAME2,
COLUMN_NAME3 HAVING COUNT(*)>1;

Query:

SELECT OFFICER_NAME, TEAM_SIZE,
POSTING_LOCATION, COUNT(*) AS QTY
FROM POSTINGS
GROUP BY OFFICER_NAME, TEAM_SIZE,
POSTING_LOCATION HAVING COUNT(*)>1;

Output:

Step 8: Find duplicates in 2(multiple) columns i.e. in TEAM_SIZE and POSTING_LOCATION in the table POSTINGS.

Query:

SELECT TEAM_SIZE, POSTING_LOCATION, 
COUNT(*) AS QTY
FROM POSTINGS
GROUP BY TEAM_SIZE, POSTING_LOCATION
 HAVING COUNT(*)>1;

Output:

Step 9: Find duplicates in 2(multiple) columns i.e. in OFFICER_NAME and TEAM_SIZE in the table POSTINGS.

Query:

SELECT OFFICER_NAME, TEAM_SIZE, 
COUNT(*) AS QTY
FROM POSTINGS
GROUP BY OFFICER_NAME, 
TEAM_SIZE HAVING COUNT(*)>1;

Output:

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