How to use sql statement distinct

The DISTINCT keyword in SQL is used to remove duplicate rows from query results. It keeps unique rows for each table and discards duplicate values. When used, follow the result column immediately at the beginning of the SELECT statement, for example: SELECT DISTINCT column name 1, column name 2 FROM table name; the benefits of DISTINCT include eliminating duplicate values, simplifying data analysis, improving query performance, and ensuring uniqueness of results. But it only removes duplicate rows, does not affect the in-line field sorting, and may cause a degradation in query performance.

Use of DISTINCT keyword in SQL

What is DISTINCT?

The DISTINCT keyword is used to remove duplicate rows from SQL query results. It keeps unique rows for each table in the query and discards duplicate values.

How to use DISTINCT?

The DISTINCT keyword is used to start the SELECT statement, followed by the query result column:

 <code class="sql">SELECT DISTINCT 列名1, 列名2 FROM 表名;</code>

For example:

Suppose there is the following table:

 <code>| id | name | |---|---| | 1 | John | | 2 | Mary | | 3 | John |</code>

The following query uses the DISTINCT keyword to delete duplicate lines:

 <code class="sql">SELECT DISTINCT name FROM 表名;</code>

Output:

 <code>| name | |---|---| | John | | Mary |</code>

Benefits of DISTINCT:

Using DISTINCT can simplify data analysis because it:

• Eliminate duplicate values ​​to make the result more concise.
• Improve query performance because the database does not have to process duplicate data.
• Make sure that only unique values ​​are included in the result.

Notes:

• DISTINCT Only duplicate rows are deleted and does not affect the sort of fields within the row.
• If the ALL keyword is not used in the query, DISTINCT is enabled by default.
• In some cases, using DISTINCT can result in a degradation in query performance because it requires additional processing of the data.

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