How to use mysql distinct

In mysql, the distinct keyword is used to filter duplicate data. It can filter duplicate data in one or more fields in the data table and only return one piece of data to the user; the distinct keyword needs to be Used together with the SELECT statement, the syntax is "SELECT DISTINCT column1, column2, ... FROM table_name;".

The operating environment of this tutorial: windows7 system, mysql8 version, Dell G3 computer.

When using the SELECT statement to perform a simple data query in MySQL, all matching records are returned. If some fields in a table do not have unique constraints, duplicate values ​​may exist in these fields. In order to query unique data, MySQL provides the DISTINCT keyword.

The main function of the DISTINCT keyword is to filter duplicate data in one or more fields in the data table, and only return one piece of data to the user.

The syntax format of the DISTINCT keyword is:

SELECT DISTINCT column1, column2, ...
FROM table_name;

where "column" is the name of one or more fields that need to eliminate duplicate records , separate multiple fields with commas.

You need to pay attention to the following points when using the DISTINCT keyword:

• The DISTINCT keyword can only be used in a SELECT statement.

• When deduplicating one or more fields, the DISTINCT keyword must be at the front of all fields.

• If there are multiple fields after the DISTINCT keyword, the multiple fields will be combined and deduplicated. That is to say, only when the combination of multiple fields is exactly the same. will be deduplicated.

Demo Database

The following is a selection of the "Customers" table in the Northwind sample database:

CustomerID	CustomerName	ContactName	Address	City	PostalCode	Country
1	Alfreds Futterkiste	Maria Anders	Obere Str. 57	Berlin	12209	Germany
2	Ana Trujillo Emparedados y helados	Ana Trujillo	Avda. de la Constituciуn 2222	Mйxico D.F.	05021	Mexico
3	Antonio Moreno Taquerнa	Antonio Moreno	Mataderos 2312	Mйxico D.F.	05023	Mexico
4	Around the Horn	Thomas Hardy	120 Hanover Sq.	London	WA1 1DP	UK
5	Berglunds snabbkцp	Christina Berglund	Berguvsvдgen 8	Luleе	S-958 22	Sweden

No SELECT instance using DISTINCT

The following SQL statement selects ALL (including duplicates) values ​​from the "Country" column in the "Customers" table:

SELECT Country FROM Customers;

##SELECT DISTINCT instance

1. Select DISTINCT values ​​only from the "Country" column in the "Customers" table:

SELECT DISTINCT Country FROM Customers;

2. List different "Countries" Quantity:

SELECT COUNT(DISTINCT Country) FROM Customers;

NOTE: The above example does not work in Firefox and Microsoft Edge! Because Microsoft Access database does not support COUNT (DISTINCT column_name). Firefox and Microsoft Edge use Microsoft Access in our example.

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