这一节主要是对于一些特定类型的优化查询: (1)count查询优化; (2)关联查询 (3)子查询 (4)GROUP BY 和 DISTINCT优化 (5)LIMIT 分页优化count查询优化COUNT()聚合函数的作用: (1)统计某一个列值的数量,也可以统计行数。需要注意的是统计列值时要求列值是非空的(不统计NULL)(2)统计结果集的行数。当列值不可能为空时统计的就是表的行数。但是为了确保一定要使用COUNT()获取结果集的行数。通配符会直接忽略所有列值直接计算行数,进行优化。对于MyISAM存储引擎,当在单表中没有限定where查询条件时COUNT(*)是非常快的,因为MyISAM本身已经存了这个行数总值。当存在where限定条件,也是需要进行查询统计的。下面给出一个简单优化的使用示例: (
简介:这一节主要是对于一些特定类型的优化查询: (1)count查询优化; (2)关联查询 (3)子查询 (4)GROUP BY 和 DISTINCT优化 (5)LIMIT 分页优化count查询优化COUNT()聚合函数的作用: (1)统计某一个列值的数量,也可以统计行数。需要注意的是统计列值时要求列值是非空的(不统计NULL)(2)统计结果集的行数。当列值不可能为空时统计的就是表的行数。但是为了确保一定要使用COUNT()获取结果集的行数。通配符会直接忽略所有列值直接计算行数,进行优化。
简介:这一节主要是对于一些特定类型的优化查询: (1)count查询优化; (2)关联查询 (3)子查询 (4)GROUP BY 和 DISTINCT优化 (5)LIMIT 分页优化count查询优化COUNT()聚合函数的作用: (1)统计某一个列值的数量,也可以统计行数。需要注意的是统计列值时要求列值是非空的(...
简介:这一节主要是对于一些特定类型的优化查询: (1)count查询优化; (2)关联查询 (3)子查询 (4)GROUP BY 和 DISTINCT优化 (5)LIMIT 分页优化count查询优化COUNT()聚合函数的作用: (1)统计某一个列值的数量,也可以统计行数。需要注意的是统计列值时要求列值是非空的(...
简介:array_count_values计算一维数组中所有值出现的个数$array = array(1, "hello", 1, "world", "hello");print_r(array_count_values($array));输出:Array( [1] => 2 &nb...
5. MySQL中count(), group by, order by的具体使用详解
简介:mysql中order by 排序查询、asc升序、desc降序,group by 分组查询、having 只能用于group by子句、作用于组内,having条件子句可以直接跟函数表达式。使用group by 子句的查询语句需要使用聚合函数。
6. mysql uses joins with aggregate functions
##Introduction: Aggregation functions are used to summarize data. Although all examples of aggregate functions so far have only summarized data from a single table, these functions can also be used with joins. To illustrate this, consider an example. If you want to retrieve all customers and the number of orders placed by each customer, the following code using the COUNT() function can complete the work:
7. How to get an array in PHP Example of the number of elements in
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