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Earlier than immediately leaping into the queries, that you must outline standards to seek out duplicate data in a desk. There will be situations when sure values in a single column are duplicated or your entire report i.e. values in all of the columns in a selected row are duplicated within the desk.
You’ll discover each the probabilities and the methods to cope with such duplicated data on this fast learn.
The simplest approach to determine the duplicated data is to easily rely what number of occasions every report seems within the desk. And the report which seems greater than as soon as is duplicated.
The operate GROUP BY is broadly utilized in SQL for knowledge aggregation. It means you possibly can group the data primarily based on values in a single or a number of columns and get aggregated values akin to rely, or sum of different columns.
Maintaining this in thoughts, let’s discover how you will discover out duplicated values in a single column.
Discover duplicate values in a single column
There will be conditions when duplicate values are current solely in a single column. The explanation for such duplicate data will be so simple as human error whereas making the information entry or updating the database.
Let’s take an instance from the orders desk and discover out which OrderID are duplicated. As that you must rely — what number of occasions every OrderID appeared within the desk — you need to group the data by OrderID as proven under.
SELECT OrderID
, COUNT(*) as occurrences
FROM orders
GROUP BY OrderID
The highlighted data (OrderIDs) occurred within the dataset greater than as soon as i.e. these are duplicated.
Nonetheless, you don’t must create separate columns as seen within the above image. You possibly can immediately get the duplicated OrderIDs utilizing the HAVING clause after GROUP BY, as proven under.
SELECT OrderID
FROM orders
GROUP BY OrderID
HAVING COUNT(*) > 1;
So, you get solely the duplicated OrderIDs that are the identical because the highlighted ones within the above desk.
Equally, there will be conditions when the values within the a number of columns for a row are duplicated within the desk.
Discover Duplicate Values In A number of Columns
Though your entire row is duplicated throughout the desk, the logic stays the identical, solely the columns you point out within the GROUP BY clause change.
As a substitute of grouping the data by a single column, right here that you must group the data by a number of columns.
Let me present you the way.
Suppose you wish to see the data the place a mixture of OrderID, Amount, and Product_Category appeared a number of occasions within the desk.
SELECT OrderID
, Amount
, Product_Category
, COUNT(*) as occurrences
FROM orders
GROUP BY OrderID
, Amount
, Product_Category
On this approach, you possibly can see that the highlighted mixture of the values within the columns OrderID, Amount, and Product_Category occurred within the desk greater than as soon as.
Once more that you must merely add HAVING COUNT(*) > 1 on the finish of the question to retrieve these duplicated data.
As the method to seek out out duplicates is dependent upon the rely of the variety of occasions a report seems within the desk, you should utilize the window operate ROW_NUMBER as properly.
The window operate ROW_NUMBER() assigns a novel sequential quantity to every report within the window outlined utilizing the PARTITION_BY clause.
So, you possibly can truly outline the window utilizing the identical columns, the place you count on to have duplicated values. So, if a report seems a number of occasions, a row variety of greater than 1 can be assigned to the duplicated data.
Let’s proceed with the identical instance.
To get the data the place a mixture of OrderID, Amount, and Product_Category appeared a number of occasions within the desk, that you must outline a window utilizing these columns within the PARTITION_BY clause as proven under.
SELECT OrderID
, Amount
, Product_Category
, ROW_NUMBER() OVER (PARTITION BY OrderID, Amount, Product_Category ORDER BY OrderID) AS row_num
FROM orders
That is the way you’ll get all of the data and the corresponding row numbers partitioned by a given set of columns. So, all of the highlighted data the place row quantity is 2 are duplicated data.
You possibly can go the above total question as a sub-query to the outer SELECT assertion under to get solely the duplicated data.
SELECT OrderID
, Amount
, Product_Category
FROM (
SELECT OrderID
, Amount
, Product_Category
, ROW_NUMBER() OVER (PARTITION BY OrderID, Amount, Product_Category ORDER BY OrderID) AS row_num
FROM orders
) AS subquery
WHERE row_num > 1;
Alternatively, when you don’t wish to use the sub-query, you possibly can create a CTE and get the information from that CTE utilizing one other question as proven under.
WITH temp_orders AS
(
SELECT OrderID
, Amount
, Product_Category
, ROW_NUMBER() OVER (PARTITION BY OrderID, Amount, Product_Category ORDER BY OrderID) AS row_num
FROM orders
)SELECT OrderID
, Amount
, Product_Category
FROM temp_orders
WHERE row_num > 1;
This question will even return precisely the identical output. So the selection is yours.
To be taught extra in regards to the ROW_NUMBER(), CTE, and GROUP BY, don’t overlook to take a look at fascinating assets on the finish of this learn!
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