MYSQL下统计一个字段中出现字符串的种类和个数
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某表有字段field1,这个字段中的数据可能出现‘aaa,bbb’,‘bbb,ccc’,‘ccc,aaa’等类似形式的多条数据,现在想分类统计各种字符串类型的出现次数,例如aaa出现多少次,要怎么写SQL语句啊
不好意思,我没说清楚,aaa,bbb,ccc都是未知的,也就是说可能有很多仲情况,要求把所有出现过的字符串都列举出来并统计出现过的次数,也有可能是一条数据中只出现aaa或者bbb
这个就是解决你那问题的。自己试下先。 参考技术B select sum(case when field1 like '%aaa%' then 1 else 0 end) as aaacount,sum(case when field1 like '%bbb%' then 1 else 0 end) as bbbcount from table1;
MySQL
分组函数
用做统计使用,又称为聚合函数或统计函数。它的种类有sun、avg、max、min、count。
要求:
1、sum和avg一般用于处理数值型,max、min、count可以处理任何数据类型。
2、上述分组函数均忽略null值
3、都可以搭配distinct使用,实现去重的统计
select sum(distinct 字段) from 表;
4、count函数
count(字段):统计该字段非空值的个数
count(*):统计结果集的行数
5、和分组函数一同查询的字段,要求是group by后出现的字段
#1.简单使用
SELECT SUM(salary) FROM employees;
SELECT AVG(salary) FROM employees;
SELECT MAX(salary) FROM employees;
SELECT MIN(salary) FROM employees;
SELECT COUNT(salary) FROM employees;
SELECT SUM(salary) 和,ROUND(AVG(salary),2) 平均,MAX(salary) 最高,MIN(salary) 最低,COUNT(salary) 个数
FROM employees;
#2.参数支持哪些数据类型
SELECT SUM(last_name),AVG(last_name) FROM employees;
SELECT SUM(hiredate),AVG(hiredate) FROM employees;
SELECT MAX(last_name),MIN(last_name) FROM employees;
SELECT MAX(hiredate),MIN(hiredate) FROM employees;
SELECT COUNT(commission_pct) FROM employees;
SELECT COUNT(last_name) FROM employees;
#3.忽略null
SELECT SUM(commission_pct),AVG(commission_pct) FROM employees;
SELECT commission_pct FROM employees;
SELECT SUM(commission_pct),AVG(commission_pct),SUM(commission_pct)/35,AVG(commission_pct)/107 FROM employees;
SELECT MAX(commission_pct),MIN(commission_pct) FROM employees;
SELECT COUNT(commission_pct) FROM employees;
#4.和distinct搭配
SELECT SUM(DISTINCT salary),SUM(salary) FROM employees;
SELECT COUNT(DISTINCT salary),COUNT(salary) FROM employees;
#5.count函数详解
SELECT COUNT(salary) FROM employees;
SELECT COUNT(*) FROM employees;
SELECT COUNT(1) FROM employees;
/*
效率上:
MyISAM存储引擎,count(*)最高
InnoDB存储引擎,count(*)和count(1)效率>count(字段)
*/
#6.和分组函数一同查询的字段有限制
SELECT AVG(salary),employee_id FROM employees;
分组查询
select 分组函数,分组后的字段
from 表
【where 分组前的筛选】
group by 分组的字段
【having 分组后的筛选】
【order by 排序列表】
使用关键字 筛选的表 位置
分组前筛选 where 原始表 group by的前面
分组后筛选 having 分组后的结果 group by的后面
1.分组函数做条件肯定是放在having子句中
2.能用分组前筛选的,就优先考虑使用分组前筛选
#案例1:查询每个工种的最高工资
SELECT MAX(salary),job_id FROM employees
GROUP BY job_id;
#案例2:查询每个位置上的部门个数
SELECT COUNT(*),location_id
FROM departments
GROUP BY location_id;
#添加筛选条件
#案例1:查询邮箱中包含a字符的,每个部门的平均工资
SELECT AVG(salary),department_id FROM employees
WHERE email LIKE '%a%' GROUP BY department_id;
#案例2:查询有奖金的每个领导手下员工的最高工资
SELECT MAX(salary),manager_id FROM employees
WHERE commission_pct IS NOT NULL
GROUP BY manager_id;
#添加复杂的筛选条件
#案例1:查询哪个部门的员工个数>2
#1.查询每个部门的员工个数
SELECT COUNT(*),department_id FROM employees
GROUP BY department_id;
#2.根据1的结果进行筛选,查询哪个部门的员工个数大于2
SELECT COUNT(*),department_id FROM employees
GROUP BY department_id HAVING COUNT(*)>2;
#案例2:查询每个工种有奖金的员工的最高工资>12000的工种编号和最高工资
#1.查询每个工种有奖金的员工的最高工资
SELECT MAX(salary),job_id FROM employees
WHERE commission_pct IS NOT NULL GROUP BY job_id;
#2.根据结果继续筛选,最高工资>12000
SELECT MAX(salary), job_id FROM employees
WHERE commission_pct IS NOT NULL GROUP BY job_id
HAVING MAX(salary)>12000;
#按表达式或函数分组
#案例:按员工姓名的长度分组,查询每一组的员工个数,筛选员工个数>5
#1.查询每个长度的员工个数
SELECT COUNT(*),LENGTH(last_name) len_name
FROM employees GROUP BY LENGTH(last_name);
#2.添加筛选条件
SELECT COUNT(*) c,LENGTH(last_name) len_name
FROM employees GROUP BY len_name HAVING c>5;
#按多个字段查询,没有顺序要求
#案例:查询每个部门每个工种的员工的平均工资
SELECT AVG(salary),department_id,job_id
FROM employees GROUP BY department_id,job_id;
#添加排序
#案例:查询每个部门每个工种的员工的平均工资,按平均工资的高低查询
SELECT AVG(salary),department_id,job_id
FROM employees GROUP BY department_id,job_id
ORDER BY AVG(salary) DESC;
连接查询
又称为多表查询,当查询的数据来自多个表时,就会用到连接查询
按功能分类分为内连接、外连接、交叉连接。而内连接包括等值连接、非等值连接、自连接。外连接包括左外连接、右外连接、全外连接。
#一、内连接
/*
语法:
select 查询列表
from 表1 别名
inner join 表2 别名
on 连接条件;
inner可以省略
*/
#1、等值连接
#案例1.查询员工名、部门名
SELECT last_name,department_name FROM departments d
INNER JOIN employees e
ON e.`department_id` = d.`department_id`;
#案例2.查询名字中包含e的员工名和工种名(添加筛选)
SELECT last_name,job_title FROM employees e
INNER JOIN jobs j ON e.`job_id`= j.`job_id`
WHERE e.`last_name` LIKE '%e%';
#案例3.查询部门个数>3的城市名和部门个数,(添加分组+筛选)
#1.查询每个城市的部门个数
#2.在1结果上筛选满足条件的
SELECT city,COUNT(*) 部门个数
FROM departments d
INNER JOIN locations l
ON d.`location_id`=l.`location_id`
GROUP BY city
HAVING COUNT(*)>3;
#案例4.查询哪个部门的员工个数>3的部门名和员工个数,并按个数降序(添加排序)
#1.查询每个部门的员工个数
SELECT COUNT(*),department_name
FROM employees e
INNER JOIN departments d
ON e.`department_id`=d.`department_id`
GROUP BY department_name;
#2.在1结果上筛选员工个数>3的记录,并排序
SELECT COUNT(*) 个数,department_name
FROM employees e
INNER JOIN departments d
ON e.`department_id`=d.`department_id`
GROUP BY department_name
HAVING COUNT(*)>3
ORDER BY COUNT(*) DESC;
#案例5.查询员工名、部门名、工种名,并按部门名降序(添加三表连接)
SELECT last_name,department_name,job_title
FROM employees e
INNER JOIN departments d ON e.`department_id`=d.`department_id`
INNER JOIN jobs j ON e.`job_id` = j.`job_id`
ORDER BY department_name DESC;
#二、非等值连接
#查询员工的工资级别
SELECT salary,grade_level
FROM employees e
JOIN job_grades g
ON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal`;
#查询工资级别的个数>20的个数,并且按工资级别降序
SELECT COUNT(*),grade_level
FROM employees e
JOIN job_grades g
ON e.`salary` BETWEEN g.`lowest_sal` AND g.`highest_sal`
GROUP BY grade_level
HAVING COUNT(*)>20
ORDER BY grade_level DESC;
#三、自连接
#查询员工的名字、上级的名字
SELECT e.last_name,m.last_name
FROM employees e
JOIN employees m
ON e.`manager_id`= m.`employee_id`;
#查询姓名中包含字符k的员工的名字、上级的名字
SELECT e.last_name,m.last_name
FROM employees e
JOIN employees m
ON e.`manager_id`= m.`employee_id`
WHERE e.`last_name` LIKE '%k%';
#二、外连接
/*
应用场景:用于查询一个表中有,另一个表没有的记录
特点:
1、外连接的查询结果为主表中的所有记录
如果从表中有和它匹配的,则显示匹配的值
如果从表中没有和它匹配的,则显示null
外连接查询结果=内连接结果+主表中有而从表没有的记录
2、左外连接,left join左边的是主表
右外连接,right join右边的是主表
3、左外和右外交换两个表的顺序,可以实现同样的效果
4、全外连接=内连接的结果+表1中有但表2没有的+表2中有但表1没有的。mysql不支持
*/
#左外连接,查询男朋友 不在男神表的的女神名
SELECT b.name FROM beauty b
LEFT OUTER JOIN boys bo
ON b.`boyfriend_id` = bo.`id`
WHERE bo.`id` IS NULL;
#案例1:查询哪个部门没有员工
#左外
SELECT d.*,e.employee_id
FROM departments d
LEFT OUTER JOIN employees e
ON d.`department_id` = e.`department_id`
WHERE e.`employee_id` IS NULL;
#右外
SELECT d.*,e.employee_id
FROM employees e
RIGHT OUTER JOIN departments d
ON d.`department_id` = e.`department_id`
WHERE e.`employee_id` IS NULL;
#交叉连接,笛卡儿积
SELECT b.*,bo.* FROM beauty b
CROSS JOIN boys bo;
子查询
出现在其他语句中的select语句,成为子查询或内查询。外部的查询语句,成为主查询或外查询。
按子查询出现的位置分为select后面、from后面、where或having后面、exists后面
按结果集的行列数不同分为标量子查询(结果集只有一行一列)、列子查询(结果集只有一列多行)、行子查询(结果集只有一行多列)、表子查询(结果集多行多列)
#一、where或having后面
/*
1、标量子查询(单行子查询)
2、列子查询(多行子查询)
3、行子查询(多列多行)
特点:
①子查询放在小括号内
②子查询一般放在条件的右侧
③标量子查询,一般搭配着单行操作符使用
> < >= <= = <>
列子查询,一般搭配着多行操作符使用
in、any/some、all
④子查询的执行优先于主查询执行,主查询的条件用到了子查询的结果
*/
#1.标量子查询
#案例1:谁的工资比 Abel 高?
#①查询Abel的工资
SELECT salary
FROM employees
WHERE last_name = 'Abel';
#②查询员工的信息,满足 salary>①结果
SELECT *
FROM employees
WHERE salary>(
SELECT salary
FROM employees
WHERE last_name = 'Abel'
);
#案例2:返回job_id与141号员工相同,salary比143号员工多的员工 姓名,job_id 和工资
#①查询141号员工的job_id
SELECT job_id FROM employees
WHERE employee_id = 141;
#②查询143号员工的salary
SELECT salary FROM employees
WHERE employee_id = 143;
#③查询员工的姓名,job_id 和工资,要求job_id=①并且salary>②
SELECT last_name,job_id,salary
FROM employees
WHERE job_id = (
SELECT job_id
FROM employees
WHERE employee_id = 141
) AND salary>(
SELECT salary
FROM employees
WHERE employee_id = 143
);
#案例3:返回公司工资最少的员工的last_name,job_id和salary
#①查询公司的最低工资
SELECT MIN(salary) FROM employees;
#②查询last_name,job_id和salary,要求salary=①
SELECT last_name,job_id,salary
FROM employees
WHERE salary=(
SELECT MIN(salary)
FROM employees
);
#案例4:查询最低工资大于50号部门最低工资的部门id和其最低工资
#①查询50号部门的最低工资
SELECT MIN(salary)
FROM employees
WHERE department_id = 50;
#②查询每个部门的最低工资
SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id;
#③ 在②基础上筛选,满足min(salary)>①
SELECT MIN(salary),department_id
FROM employees
GROUP BY department_id
HAVING MIN(salary)>(
SELECT MIN(salary)
FROM employees
WHERE department_id = 50
);
#2.列子查询(多行子查询)
#IN/NOT IN:等于列表中的任意一个
#ANY/SOME:和子查询返回的某一个值比较
#ALL:和子查询返回的所有值比较
#案例1:返回location_id是1400或1700的部门中的所有员工姓名
#①查询location_id是1400或1700的部门编号
SELECT DISTINCT department_id
FROM departments
WHERE location_id IN(1400,1700);
#②查询员工姓名,要求部门号是①列表中的某一个
SELECT last_name
FROM employees
WHERE department_id <>ALL(
SELECT DISTINCT department_id
FROM departments
WHERE location_id IN(1400,1700)
);
#案例2:返回其它工种中比job_id为‘IT_PROG’工种任一工资低的员工
#的员工号、姓名、job_id 以及salary
#①查询job_id为‘IT_PROG’部门任一工资
SELECT DISTINCT salary FROM employees
WHERE job_id = 'IT_PROG';
#②查询员工号、姓名、job_id 以及salary,salary<(①)的任意一个
SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ANY(
SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';
#或
SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(
SELECT MAX(salary)
FROM employees
WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';
#案例3:返回其它部门中比job_id为‘IT_PROG’部门所有工资都低的员工的员工号、姓名、job_id 以及salary
SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<ALL(
SELECT DISTINCT salary
FROM employees
WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';
#或
SELECT last_name,employee_id,job_id,salary
FROM employees
WHERE salary<(
SELECT MIN( salary)
FROM employees
WHERE job_id = 'IT_PROG'
) AND job_id<>'IT_PROG';
#3、行子查询(结果集一行多列或多行多列)
#案例:查询员工编号最小并且工资最高的员工信息
SELECT * FROM employees
WHERE (employee_id,salary)=(
SELECT MIN(employee_id),MAX(salary)
FROM employees
);
#①查询最小的员工编号
SELECT MIN(employee_id) FROM employees;
#②查询最高工资
SELECT MAX(salary) FROM employees;
#③查询员工信息
SELECT * FROM employees
WHERE employee_id=(
SELECT MIN(employee_id)
FROM employees
)AND salary=(
SELECT MAX(salary)
FROM employees
);
#二、select后面
/*
仅仅支持标量子查询
*/
#案例:查询每个部门的员工个数
SELECT d.*,(
SELECT COUNT(*)
FROM employees e
WHERE e.department_id = d.`department_id`
) 个数
FROM departments d;
#案例2:查询员工号=102的部门名
SELECT (
SELECT department_name,e.department_id
FROM departments d
INNER JOIN employees e
ON d.department_id=e.department_id
WHERE e.employee_id=102
) 部门名;
#三、from后面
/*将子查询结果充当一张表,要求必须起别名*/
#案例:查询每个部门的平均工资的工资等级
#①查询每个部门的平均工资
SELECT AVG(salary),department_id
FROM employees GROUP BY department_id;
# ②
SELECT * FROM job_grades;
#②连接①的结果集和job_grades表,筛选条件平均工资 between lowest_sal and highest_sal
SELECT ag_dep.*,g.`grade_level`
FROM (
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id
) ag_dep
INNER JOIN job_grades g
ON ag_dep.ag BETWEEN lowest_sal AND highest_sal;
#四、exists后面(相关子查询)
#语法:exists(完整的查询语句) 结果:1或0
SELECT EXISTS(SELECT employee_id FROM employees WHERE salary=300000);
#案例1:查询有员工的部门名
#in
SELECT department_name
FROM departments d
WHERE d.`department_id` IN(
SELECT department_id
FROM employees
);
#exists
SELECT department_name
FROM departments d
WHERE EXISTS(
SELECT *
FROM employees e
WHERE d.`department_id`=e.`department_id`
);
#案例2:查询没有女朋友的男神信息
#in
SELECT bo.*
FROM boys bo
WHERE bo.id NOT IN(
SELECT boyfriend_id
FROM beauty
);
#exists
SELECT bo.*
FROM boys bo
WHERE NOT EXISTS(
SELECT boyfriend_id
FROM beauty b
WHERE bo.`id`=b.`boyfriend_id`
);
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