# count函数

## count函数相关知识

• SQL XQuery COUNT函数
本实例是演示SQL查询中XQuery使用COUNT函数。DECLARE&nbsp;@xml&nbsp;XML&nbsp;=&nbsp;&#39;&lt;Devs&gt;&nbsp;&nbsp;&nbsp;&lt;lang&nbsp;name=&quot;VB.NET&quot;/&gt;&nbsp;&nbsp;&nbsp;&lt;lang&nbsp;name=&quot;C#.NET&quot;/&gt;&nbsp;&nbsp;&nbsp;&lt;lang&nbsp;name=&quot;Java&quot;&nbsp;/&gt;&nbsp;&nbsp;&nbsp;&lt;lang&nbsp;name=&quot;Javascript&quot;&nbsp;/&gt;&nbsp;&nbsp;&nbsp;&lt;lang&nbsp;name=&quot;ASP.NET&quot;&nbsp;/&gt;&lt;/Devs&gt;&#39;SELECT&nbsp;COUNT(*)&nbsp;AS&nbsp;[To
• 详细解读MySQL中COUNT函数的用法
MySQL的COUNT函数是最简单的功能，非常有用的计算，预计由一个SELECT语句返回的记录数。要了解COUNT函数考虑的EMPLOYEE_TBL的的表具有以下记录：?mysql&gt; SELECT * FROM employee_tbl;+------+------+------------+--------------------+| id&nbsp; | name | work_date | daily_typing_pages |+------+------+------------+--------------------+|&nbsp; 1 | John | 2007-01-24 |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 250 ||&nbsp; 2 | Ram | 2007-05-27 |&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 220 ||&nbsp; 3 | Jack | 2007-05-06 |
• MySQL COUNT函数
Summary: in this tutorial, you will learn how to use the&nbsp;MySQL COUNT&nbsp;function&nbsp;to count the number rows in a table.Introduction to the MySQL COUNT functionThe&nbsp;COUNT&nbsp;function returns the number of rows in a table. The&nbsp;COUNT&nbsp;function allows you to count all rows in a table or rows that match a particular condition.The syntax of the&nbsp;COUNT&nbsp;function is as follows.COUNT(expression)The return type of the&nbsp;COUNT&nbsp
• MySQL count()函数及其优化
很简单，就是为了统计记录数由SELECT返回为了理解这个函数，让我们祭出 employee_tbl 表所有记录统计行的总数计算 Zara 的记录数注意：由于 SQL 查询对大小写不敏感，所以在 WHERE 条件中，无论是写成 ZARA 还是 Zara，结果都是一样的count(1),count(*),count(字段)区别count(1)和count(*)作用都是检索表中所有记录行的数目，不论其是否包含null值区别count(1)比count(*)效率高二 . count(字段)与count(1)和count(*)的区别count(字段)的作用是检索表中的这个字段的非空行数，不统计这个字段值为null的记录任何情况下SELECT COUNT(1) FROM tablename是最优选择尽量减少SELECT COUNT(*) FROM tablename WHERE COL = ‘value’ 这种杜绝SELECT COUNT(COL) FROM tablename WHERE COL2 = ‘value’