Master SQL DISTINCT: Deleting duplicates makes it easy

SQL DISTINCT keyword explanation: Efficiently remove duplicate lines

The DISTINCT keyword in SQL is mainly used to filter duplicate rows in query results to ensure the uniqueness of each row of data in the returned result set.

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DISTINCT working mechanism

SELECT queries sometimes return results containing duplicate rows. The purpose of the DISTINCT keyword is to remove these redundant data and retain only a single row of records of unique values ​​for each set.

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grammar

 <code class="language-sql">SELECT DISTINCT column1, column2, ... FROM table_name;</code>

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Example

1. Remove duplicate values

Suppose there is a list of employees called employees :

Employeeid	department
1	hr
2	it
3	hr
4	Sales

Perform the following query:

 <code class="language-sql">SELECT DISTINCT department FROM employees;</code>

result:

department

hr

it

Sales

As you can see, the duplicate "hr" department has been removed.

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2. Select a unique combination

Consider another order table called orders :

Orderid	customerid	productid
101	1	a
102	1	b
103	1	a
104	2	c

Perform the following query:

 <code class="language-sql">SELECT DISTINCT CustomerID, ProductID FROM Orders;</code>

result:

customerid	productid
1	a
1	b
2	c

DISTINCT removes duplicate rows according to the combination of customerid and productid .

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Application scenarios of DISTINCT

1. Get unique values : When you need to find all unique values ​​in a column or combination of columns. For example, list all the different product categories in the database.
2. Remove redundant data : In data analysis or reporting, if duplicate rows are not required. For example, get a unique department name from the employee table.
3. Data Cleaning : Used to clean up data sets and remove duplicate data.

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Limitations of DISTINCT

1. Performance Impact : DISTINCT increases query execution time, especially on large datasets, as it requires scanning and comparing all rows.
2. Conditional deduplication cannot be achieved : If you need to remove duplicate data based on a specific condition (e.g., keeping the latest row of each unique value), you need to use other techniques, such as ROW_NUMBER() function.

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Tips for Using DISTINCT

1. Use DISTINCT only if necessary, as it will affect performance.
2. For complex deduplication operations, consider using an aggregate function ( GROUP BY ) or an analytical function as an alternative.

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Summarize

The DISTINCT keyword is a concise and powerful tool in SQL to remove duplicate rows in query results, thereby ensuring the uniqueness of the result data. When using it, the performance impact should be weighed and the appropriate technology should be selected according to actual needs.

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