ORACLE 007

2024-07-18 00:27| 来源: 网络整理| 查看: 265

在介绍GROUP BY 和 HAVING 子句前，我们必需先讲讲sql语言中一种特殊的函数：聚合函数，例如SUM, COUNT, MAX, AVG等。这些函数和其它函数的根本区别就是它们一般作用在多条记录上。

SELECT SUM(population) FROM bbc 　　这里的SUM作用在所有返回记录的population字段上，结果就是该查询只返回一个结果，即所有国家的总人口数。

　　通过使用GROUP BY 子句，可以让SUM 和 COUNT 这些函数对属于一组的数据起作用。当你指定 GROUP BY region 时，属于同一个region（地区）的一组数据将只能返回一行值，也就是说，表中所有除region（地区）外的字段，只能通过 SUM, COUNT等聚合函数运算后返回一个值。

　　HAVING子句可以让我们筛选成组后的各组数据，WHERE子句在聚合前先筛选记录．也就是说作用在GROUP BY 子句和HAVING子句前．而 HAVING子句在聚合后对组记录进行筛选。

　　让我们还是通过具体的实例来理解GROUP BY 和 HAVING 子句，还采用第三节介绍的bbc表。

　　SQL实例：

　　一、显示每个地区的总人口数和总面积：

SELECT region, SUM(population), SUM(area)FROM bbcGROUP BY region 　　先以region把返回记录分成多个组，这就是GROUP BY的字面含义。分完组后，然后用聚合函数对每组中的不同字段（一或多条记录）作运算。

　　二、显示每个地区的总人口数和总面积．仅显示那些面积超过1000000的地区。

SELECT region, SUM(population), SUM(area)FROM bbcGROUP BY regionHAVING SUM(area)>1000000 　　在这里，我们不能用where来筛选超过1000000的地区，因为表中不存在这样一条记录。

　　相反，HAVING子句可以让我们筛选成组后的各组数据．

group by 的增强、高级子查询、DML和DDL语句扩展

组函数avg,sum,count,min,max select id,sum(age) from users group by id having sum(age)>40; 超级聚合rollup,cuberollup从右往左再聚合select id,num,sum(age) from users group by rollup(id,num)cube从左往右取select id,num,sum(age) from users group by cube(id,num) grouping函数,返回的值，1肯定，0否定 select id,sum(age),grouping(id) from users; grouping sets分别求统计，再unionselect id,num,sum(age) from users group by grouping sets((id),(num)) select id,num,sum(age) from users group by rollup((id,num)); select id,num,sum(age) from users group by id,rollup(num); 高级子查询 inner query ,outer query select name from users where age >(select age from users where id =322); 成对的进行比较 select student_id,teacher_id,class_id from studentswhere (teacher_id,class_id) in (select teacher_id,class_id from students where student_id in (222,332)) 在from中使用子查询,注意要加上别名 select a.id,a.name,b.type from students a,(select type from teachers) b where a.teacher_id=b.id; 标量子查询精确的从一行中返回一列,独立的关联子查询循环的方式加以执行，内查询依赖于外查询select name,age from users awhere age >(select avg(age) from users b where b.id=a.id ) exists,not exists select * from students t1 where exists (select * from student_details t2 where t1.id=t2.id and t2.detail_id=22); 关联更新用另外一张表更新当前表 update test1 t2 set age= (select age from test2 t2 where t1.id=t2.id) 关联删除 delete from test1 t1 where id in (select id from test2 t2 where t1.id=t2.id and age >20)) 分层结构select id,name from student start with id=292 connect by prior id=c_id; select level,id,name from student start with id=292 connect by prior id=c_id; DML和DDL语句扩展 DML:INSERT,DELETE,UPDATE 多表插入无条件insert有条件all insert有条件first insert选择insert insert [all] [conditional_insert_clause][insert_into_clause values_clause] (subquery) insert all into test1(id,name) values(id,name) into test2(id,address) values(id,address) select id,name,address from test3 where id>23; 有条件的插入insert all when name like 't%' theninto test1(id,name) values(id,name) when name like 'h%' theninto test2(id,address) values(id,address) select id,name,address from test3 where id>23; 旋转插入假设表sales_source_date的表结构为,employee_id,week_id,sales_mon,sales_tue,sales_wed,sales_thu,sales_fri我想把这些记录保存在sales_info表中，表结构为employee_id,week,salesinsert allinto sales_info values (employee_id,week_id,sales_mon)into sales_info values (employee_id,week_id,sales_tue)into sales_info values (employee_id,week_id,sales_wed)into sales_info values (employee_id,week_id,sales_thu)into sales_info values (employee_id,week_id,sales_fri)select employee_id,week_id,sales_mon,sales_tue,sales_wed,sales_thu,sales_fri from sales_source_date; 创建表时创建索引create table test_index_table(id int not null primary key using index(create index index1 on test_index_table(id)),

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