2.2 SELECT DISTINCT 语句

MySQL DISTINCT 子句简介

从表中查询数据时，可能会收到重复的行记录。为了删除这些重复行，可以在SELECT语句中使用DISTINCT子句。

DISTINCT子句的语法如下：

SELECT DISTINCT
    columns
FROM
    table_name
WHERE
    where_conditions;

MySQL DISTINCT 示例

下面来看看一个使用DISTINCT子句从employees表中选择员工的唯一姓氏(lastName)的简单示例。

首先，使用SELECT语句从employees表中查询员工的姓氏(lastName)，如下所示：

SELECT 
    lastname
FROM
    employees
ORDER BY lastname;

执行上面查询语句，得到以下结果

mysql> SELECT lastname FROM employees ORDER BY lastname;
+-----------+
| lastname  |
+-----------+
| Bondur    |
| Bondur    |
| Bott      |
| Bow       |
| Castillo  |
| Firrelli  |
| Firrelli  |
| Fixter    |
| Gerard    |
| Hernandez |
| Jennings  |
| Jones     |
| Kato      |
| King      |
| Marsh     |
| Murphy    |
| Nishi     |
| Patterson |
| Patterson |
| Patterson |
| Thompson  |
| Tseng     |
| Vanauf    |
+-----------+
23 rows in set

可看到上面结果中，有好些结果是重复的，比如：Bondur，Firrelli等，那如何做到相同的结果只显示一个呢？要删除重复的姓氏，请将DISTINCT子句添加到SELECT语句中，如下所示：

SELECT DISTINCT
    lastname
FROM
    employees
ORDER BY lastname;

执行上面查询，得到以下输出结果

mysql> SELECT DISTINCT lastname FROM employees ORDER BY lastname;
+-----------+
| lastname  |
+-----------+
| Bondur    |
| Bott      |
| Bow       |
| Castillo  |
| Firrelli  |
| Fixter    |
| Gerard    |
| Hernandez |
| Jennings  |
| Jones     |
| Kato      |
| King      |
| Marsh     |
| Murphy    |
| Nishi     |
| Patterson |
| Thompson  |
| Tseng     |
| Vanauf    |
+-----------+
19 rows in set

当使用DISTINCT子句时，重复的姓氏(lastname)在结果集中被消除。

MySQL DISTINCT和NULL值

如果列具有NULL值，并且对该列使用DISTINCT子句，MySQL将保留一个NULL值，并删除其它的NULL值，因为DISTINCT子句将所有NULL值视为相同的值。

例如，在customers表中，有很多行的州(state)列是NULL值。当使用DISTINCT子句来查询客户所在的州时，我们将看到唯一的州和NULL值，如下查询所示：

SELECT DISTINCT
    state
FROM
    customers;

执行上面查询语句后，输出结果如下 -

mysql> SELECT DISTINCT state FROM customers;
+---------------+
| state         |
+---------------+
| NULL          |
| NV            |
| Victoria      |
| CA            |
| NY            |
| PA            |
| CT            |
| MA            |
| Osaka         |
| BC            |
| Qubec         |
| Isle of Wight |
| NSW           |
| NJ            |
| Queensland    |
| Co. Cork      |
| Pretoria      |
| NH            |
| Tokyo         |
+---------------+
19 rows in set

MySQL DISTINCT 在多列上的使用

可以使用具有多个列的DISTINCT子句。在这种情况下，MySQL 使用所有列的组合来确定结果集中行的唯一性。

例如，要从customers表中获取城市(city)和州(state)的唯一组合，可以使用以下查询：

SELECT DISTINCT
    state, city
FROM
    customers
WHERE
    state IS NOT NULL
ORDER BY state , city;

执行上面查询，得到以下结果 -

mysql> SELECT DISTINCT state, city FROM  customers WHERE state IS NOT NULL ORDER BY state ,city;
+---------------+----------------+
| state         | city           |
+---------------+----------------+
| BC            | Tsawassen      |
| BC            | Vancouver      |
| CA            | Brisbane       |
| CA            | Burbank        |
| CA            | Burlingame     |
| CA            | Glendale       |
| CA            | Los Angeles    |
| CA            | Pasadena       |
| CA            | San Diego      |
| CA            | San Francisco  |
| CA            | San Jose       |
| CA            | San Rafael     |
| Co. Cork      | Cork           |
| CT            | Bridgewater    |
| CT            | Glendale       |
| CT            | New Haven      |
| Isle of Wight | Cowes          |
| MA            | Boston         |
| MA            | Brickhaven     |
| MA            | Cambridge      |
| MA            | New Bedford    |
| NH            | Nashua         |
| NJ            | Newark         |
| NSW           | Chatswood      |
| NSW           | North Sydney   |
| NV            | Las Vegas      |
| NY            | NYC            |
| NY            | White Plains   |
| Osaka         | Kita-ku        |
| PA            | Allentown      |
| PA            | Philadelphia   |
| Pretoria      | Hatfield       |
| Qubec         | Montral        |
| Queensland    | South Brisbane |
| Tokyo         | Minato-ku      |
| Victoria      | Glen Waverly   |
| Victoria      | Melbourne      |
+---------------+----------------+
37 rows in set

没有DISTINCT子句，将查询获得州(state)和城市(city)的重复组合如下：

SELECT 
    state, city
FROM
    customers
WHERE
    state IS NOT NULL
ORDER BY state, city;

执行上面查询，得到以下结果 -

DISTINCT 子句与 GROUP BY 子句比较

如果在SELECT语句中使用GROUP BY子句，而不使用聚合函数，则GROUP BY子句的行为与DISTINCT子句类似。

以下语句使用GROUP BY子句来选择customers表中客户的唯一state列的值。

SELECT 
    state
FROM
    customers
GROUP BY state;

执行上面查询，得到以下结果 -

mysql> SELECT  state FROM customers GROUP BY state;
+---------------+
| state         |
+---------------+
| NULL          |
| BC            |
| CA            |
| Co. Cork      |
| CT            |
| Isle of Wight |
| MA            |
| NH            |
| NJ            |
| NSW           |
| NV            |
| NY            |
| Osaka         |
| PA            |
| Pretoria      |
| Qubec         |
| Queensland    |
| Tokyo         |
| Victoria      |
+---------------+
19 rows in set

可以通过使用DISTINCT子句来实现类似的结果：

mysql> SELECT DISTINCT state FROM customers;
+---------------+
| state         |
+---------------+
| NULL          |
| NV            |
| Victoria      |
| CA            |
| NY            |
| PA            |
| CT            |
| MA            |
| Osaka         |
| BC            |
| Qubec         |
| Isle of Wight |
| NSW           |
| NJ            |
| Queensland    |
| Co. Cork      |
| Pretoria      |
| NH            |
| Tokyo         |
+---------------+
19 rows in set

一般而言，DISTINCT子句是GROUP BY子句的特殊情况。 DISTINCT子句和GROUP BY子句之间的区别是GROUP BY子句可对结果集进行排序，而DISTINCT子句不进行排序。

如果将ORDER BY子句添加到使用DISTINCT子句的语句中，则结果集将被排序，并且与使用GROUP BY子句的语句返回的结果集相同。

SELECT DISTINCT
    state
FROM
    customers
ORDER BY state;

执行上面查询，得到以下结果 -

mysql> SELECT DISTINCT state FROM customers ORDER BY state;
+---------------+
| state         |
+---------------+
| NULL          |
| BC            |
| CA            |
| Co. Cork      |
| CT            |
| Isle of Wight |
| MA            |
| NH            |
| NJ            |
| NSW           |
| NV            |
| NY            |
| Osaka         |
| PA            |
| Pretoria      |
| Qubec         |
| Queensland    |
| Tokyo         |
| Victoria      |
+---------------+
19 rows in set

MySQL DISTINCT 和聚合函数

可以使用具有聚合函数(例如SUM，AVG和COUNT)的DISTINCT子句中，在MySQL将聚合函数应用于结果集之前删除重复的行。

例如，要计算美国客户的唯一state列的值，可以使用以下查询：

SELECT 
    COUNT(DISTINCT state)
FROM
    customers
WHERE
    country = 'USA';

执行上面查询，得到以下结果 -

mysql> SELECT  COUNT(DISTINCT state) FROM customers WHERE country = 'USA';
+-----------------------+
| COUNT(DISTINCT state) |
+-----------------------+
|                     8 |
+-----------------------+
1 row in set

MySQL DISTINCT与LIMIT子句

如果要将DISTINCT子句与 LIMIT 子句一起使用，MySQL 会在查找LIMIT子句中指定的唯一行数时立即停止搜索。

以下查询customers表中的前3个非空(NOT NULL)唯一state列的值。

mysql> SELECT DISTINCT state FROM customers WHERE state IS NOT NULL LIMIT 3;
+----------+
| state    |
+----------+
| NV       |
| Victoria |
| CA       |
+----------+
3 rows in set

在本教程中，我们学习了使用 MySQL DISTINCT子句的各种方法，例如消除重复行和计数非NULL值。

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