d单行函数
#常见函数/* 调用 select 函数名(实参列表) form 表 分类: 单行函数: concat,length,ifnull 分组函数:*/ show VARIABLES like '%char%';#字符函数 #1 LENGTH(str) select LENGTH('张三丰hahaha'); #2 concat select CONCAT(last_name,'_',first_name) from employees; #3 upper lower select UPPER('abc'); select LOWER('ABC'); #4 substr select SUBSTR('1234567',3); select SUBSTR('1234567',1,3); #5 instr 返回起始索引 select INSTR('1234567','456'); #6 trim select TRIM(' str '); select trim('a' from 'aa11aa11aa'); #7 lpad,rpad 用指定的字符实现左右填充指定长度 select LPAD('123456',10,'*1'); select RPAD('123456',10,'*'); #8 replace select REPLACE('123456','123','321');#数学函数 #1 round select ROUND(1.6); select ROUND(1.567,2); #2 CEIL floor向上,下取整 SELECT CEIL(1.002); select floor(9.9); #3 truncate 截断 select TRUNCATE(1.2345,1); #4 mod select mod(10,3);#日期函数 #1 NOW() select now(); #2 curdate 返回当前系统日期,不包含时间 select CURDATE(); #3 curtime 返回当前时间,不包含日期 select CURTIME(); #4 获取指定时间的部分 select year(now()); #5 str_to_date 将日期格式的字符转换成指定格式的日期 select STR_TO_DATE('9-13-1999','%m-%d-%Y'); #6 date_format 将日期转换成字符 select DATE_FORMAT('2018/6/6','%Y年%m月%d日');#其他函数 SELECT VERSION(); select DATABASE(); select user();#流程控制函数 #1 if select if(10<5,'大','小');----------------------- #2 case 函数的使用一:相当于JAVA中的switch /* case 要判断的字段表达式 when 常量1 then 要显示的值1或语句1; when 常量2 then 要显示的值2或语句2; 。。。 else end *//* 案例:查询员工的工资,要求 部门号=30,显示的工资1.1倍 部门号=40,显示的工资1.2倍 部门号=50,显示的工资1.3倍*/select salary 原始工资,department_id,case department_idwhen 30 then salary*1.1when 40 then salary*1.2when 50 then salary*1.3else salaryend as 新工资from employees; #2 case函数的使用二 类似于多重if /* CASE when 条件1 then 要显示的值1或 语句1 when 条件2 then 要显示的值2或 语句2 else end */
分组函数
#二、分组函数/*功能:用作统计使用,又称为聚合函数或统计函数或组函数分类:sum 求和、avg 平均值、max 最大值 、min 最小值 、count 计算个数特点:1、sum、avg一般用于处理数值型 max、min、count可以处理任何类型2、以上分组函数都忽略null值3、可以和distinct搭配实现去重的运算4、count函数的单独介绍一般使用count(*)用作统计行数5、和分组函数一同查询的字段要求是group by后的字段*/#1、简单 的使用SELECT SUM(salary) FROM employees;SELECT AVG(salary) FROM employees;SELECT MIN(salary) FROM employees;SELECT MAX(salary) FROM employees;SELECT COUNT(salary) FROM employees;SELECT SUM(salary) 和,AVG(salary) 平均,MAX(salary) 最高,MIN(salary) 最低,COUNT(salary) 个数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、是否忽略nullSELECT SUM(commission_pct) ,AVG(commission_pct),SUM(commission_pct)/35,SUM(commission_pct)/107 FROM employees;SELECT MAX(commission_pct) ,MIN(commission_pct) FROM employees;SELECT COUNT(commission_pct) FROM employees;SELECT 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函数的详细介绍SELECT COUNT(salary) FROM employees;SELECT COUNT(*) FROM employees;SELECT COUNT(1) FROM employees;效率:MYISAM存储引擎下 ,COUNT(*)的效率高INNODB存储引擎下,COUNT(*)和COUNT(1)的效率差不多,比COUNT(字段)要高一些#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
/*语法:select 查询列表from 表【where 筛选条件】group by 分组的字段【order by 排序的字段】;特点:1、和分组函数一同查询的字段必须是group by后出现的字段2、筛选分为两类:分组前筛选和分组后筛选 针对的表 位置 连接的关键字分组前筛选 原始表 group by前 where 分组后筛选 group by后的结果集 group by后 having问题1:分组函数做筛选能不能放在where后面答:不能问题2:where——group by——having一般来讲,能用分组前筛选的,尽量使用分组前筛选,提高效率3、分组可以按单个字段也可以按多个字段4、可以搭配着排序使用*/#引入:查询每个部门的员工个数SELECT COUNT(*) FROM employees WHERE department_id=90;#1.简单的分组#案例1:查询每个工种的员工平均工资SELECT AVG(salary),job_idFROM employeesGROUP BY job_id;#案例2:查询每个位置的部门个数SELECT COUNT(*),location_idFROM departmentsGROUP BY location_id;#2、可以实现分组前的筛选#案例1:查询邮箱中包含a字符的 每个部门的最高工资SELECT MAX(salary),department_idFROM employeesWHERE 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平均工资SELECT AVG(salary),manager_idFROM employeesWHERE commission_pct IS NOT NULLGROUP BY manager_id;#3、分组后筛选#案例:查询哪个部门的员工个数>5#①查询每个部门的员工个数SELECT COUNT(*),department_idFROM employeesGROUP BY department_id;#② 筛选刚才①结果SELECT COUNT(*),department_idFROM employeesGROUP BY department_idHAVING COUNT(*)>5;#案例2: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SELECT job_id,MAX(salary)FROM employeesWHERE commission_pct IS NOT NULLGROUP BY job_idHAVING MAX(salary)>12000;#案例3:领导编号>102的每个领导手下的最低工资大于5000的领导编号和最低工资manager_id>102SELECT manager_id,MIN(salary)FROM employeesGROUP BY manager_idHAVING MIN(salary)>5000;#4.添加排序#案例:每个工种有奖金的员工的最高工资>6000的工种编号和最高工资,按最高工资升序SELECT job_id,MAX(salary) mFROM employeesWHERE commission_pct IS NOT NULLGROUP BY job_idHAVING m>6000ORDER BY m ;#5.按多个字段分组#案例:查询每个工种每个部门的最低工资,并按最低工资降序SELECT MIN(salary),job_id,department_idFROM employeesGROUP BY department_id,job_idORDER BY MIN(salary) DESC;
连接查询92
#进阶6:连接查询/*含义:又称多表查询,当查询的字段来自于多个表时,就会用到连接查询笛卡尔乘积现象:表1 有m行,表2有n行,结果=m*n行发生原因:没有有效的连接条件如何避免:添加有效的连接条件分类: 按年代分类: sql92标准:仅仅支持内连接 sql99标准【推荐】:支持内连接+外连接(左外和右外)+交叉连接 按功能分类: 内连接: 等值连接 非等值连接 自连接 外连接: 左外连接 右外连接 全外连接 交叉连接*/SELECT * FROM beauty;SELECT * FROM boys;SELECT NAME,boyName FROM boys,beautyWHERE beauty.boyfriend_id= boys.id;#一、sql92标准#1、等值连接/*① 多表等值连接的结果为多表的交集部分②n表连接,至少需要n-1个连接条件③ 多表的顺序没有要求④一般需要为表起别名⑤可以搭配前面介绍的所有子句使用,比如排序、分组、筛选*/#案例1:查询女神名和对应的男神名SELECT NAME,boyName FROM boys,beautyWHERE beauty.boyfriend_id= boys.id;#案例2:查询员工名和对应的部门名SELECT last_name,department_nameFROM employees,departmentsWHERE employees.`department_id`=departments.`department_id`;#2、为表起别名/*①提高语句的简洁度②区分多个重名的字段注意:如果为表起了别名,则查询的字段就不能使用原来的表名去限定*/#查询员工名、工种号、工种名SELECT e.last_name,e.job_id,j.job_titleFROM employees e,jobs jWHERE e.`job_id`=j.`job_id`;#3、两个表的顺序是否可以调换#查询员工名、工种号、工种名SELECT e.last_name,e.job_id,j.job_titleFROM jobs j,employees eWHERE e.`job_id`=j.`job_id`;#4、可以加筛选#案例:查询有奖金的员工名、部门名SELECT last_name,department_name,commission_pctFROM employees e,departments dWHERE e.`department_id`=d.`department_id`AND e.`commission_pct` IS NOT NULL;#案例2:查询城市名中第二个字符为o的部门名和城市名SELECT department_name,cityFROM departments d,locations lWHERE d.`location_id` = l.`location_id`AND city LIKE '_o%';#5、可以加分组#案例1:查询每个城市的部门个数SELECT COUNT(*) 个数,cityFROM departments d,locations lWHERE d.`location_id`=l.`location_id`GROUP BY city;#案例2:查询有奖金的每个部门的部门名和部门的领导编号和该部门的最低工资SELECT department_name,d.`manager_id`,MIN(salary)FROM departments d,employees eWHERE d.`department_id`=e.`department_id`AND commission_pct IS NOT NULLGROUP BY department_name,d.`manager_id`;#6、可以加排序#案例:查询每个工种的工种名和员工的个数,并且按员工个数降序SELECT job_title,COUNT(*)FROM employees e,jobs jWHERE e.`job_id`=j.`job_id`GROUP BY job_titleORDER BY COUNT(*) DESC;#7、可以实现三表连接?#案例:查询员工名、部门名和所在的城市SELECT last_name,department_name,cityFROM employees e,departments d,locations lWHERE e.`department_id`=d.`department_id`AND d.`location_id`=l.`location_id`AND city LIKE 's%'ORDER BY department_name DESC;#2、非等值连接#案例1:查询员工的工资和工资级别SELECT salary,grade_levelFROM employees e,job_grades gWHERE salary BETWEEN g.`lowest_sal` AND g.`highest_sal`AND g.`grade_level`='A';/*select salary,employee_id from employees;select * from job_grades;CREATE TABLE job_grades(grade_level VARCHAR(3), lowest_sal int, highest_sal int);INSERT INTO job_gradesVALUES ('A', 1000, 2999);INSERT INTO job_gradesVALUES ('B', 3000, 5999);INSERT INTO job_gradesVALUES('C', 6000, 9999);INSERT INTO job_gradesVALUES('D', 10000, 14999);INSERT INTO job_gradesVALUES('E', 15000, 24999);INSERT INTO job_gradesVALUES('F', 25000, 40000);*/#3、自连接#案例:查询 员工名和上级的名称SELECT e.employee_id,e.last_name,m.employee_id,m.last_nameFROM employees e,employees mWHERE e.`manager_id`=m.`employee_id`;
连接查询99
#二、sql99语法/*语法: select 查询列表 from 表1 别名 【连接类型】 join 表2 别名 on 连接条件 【where 筛选条件】 【group by 分组】 【having 筛选条件】 【order by 排序列表】 分类:内连接(★):inner外连接 左外(★):left 【outer】 右外(★):right 【outer】 全外:full【outer】交叉连接:cross */#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;分类:等值非等值自连接特点:①添加排序、分组、筛选②inner可以省略③ 筛选条件放在where后面,连接条件放在on后面,提高分离性,便于阅读④inner join连接和sql92语法中的等值连接效果是一样的,都是查询多表的交集*/#1、等值连接#案例1.查询员工名、部门名SELECT last_name,department_nameFROM departments d JOIN employees eON 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_titleFROM employees eINNER JOIN jobs jON e.`job_id`= j.`job_id`WHERE e.`last_name` LIKE '%e%';#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 dINNER JOIN locations lON d.`location_id`=l.`location_id`GROUP BY cityHAVING 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_nameFROM employees eINNER JOIN departments dON 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_nameFROM employees eINNER JOIN departments dON e.`department_id`=d.`department_id`GROUP BY department_nameHAVING COUNT(*)>3ORDER BY COUNT(*) DESC;#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_titleFROM employees eINNER 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_levelFROM 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_levelFROM 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 */ #引入:查询男朋友 不在男神表的的女神名 SELECT * FROM beauty; SELECT * FROM boys; #左外连接 SELECT b.*,bo.* FROM boys bo LEFT OUTER JOIN beauty b ON b.`boyfriend_id` = bo.`id` WHERE b.`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; #全外 USE girls; SELECT b.*,bo.* FROM beauty b FULL OUTER JOIN boys bo ON b.`boyfriend_id` = bo.id; #交叉连接 SELECT b.*,bo.* FROM beauty b CROSS JOIN boys bo; #sql92和 sql99pk /* 功能:sql99支持的较多 可读性:sql99实现连接条件和筛选条件的分离,可读性较高 */
子查询
#进阶7:子查询/*含义:出现在其他语句中的select语句,称为子查询或内查询外部的查询语句,称为主查询或外查询分类:按子查询出现的位置: select后面: 仅仅支持标量子查询 from后面: 支持表子查询 where或having后面:★ 标量子查询(单行) √ 列子查询 (多行) √ 行子查询 exists后面(相关子查询) 表子查询按结果集的行列数不同: 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(结果集有一行多列) 表子查询(结果集一般为多行多列)*/#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、all④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 salaryFROM employeesWHERE last_name = 'Abel'#②查询员工的信息,满足 salary>①结果SELECT *FROM employeesWHERE 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_idSELECT job_idFROM employeesWHERE employee_id = 141#②查询143号员工的salarySELECT salaryFROM employeesWHERE employee_id = 143#③查询员工的姓名,job_id 和工资,要求job_id=①并且salary>②SELECT last_name,job_id,salaryFROM employeesWHERE 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=①SELECT last_name,job_id,salaryFROM employeesWHERE 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 employeesWHERE department_id = 50#②查询每个部门的最低工资SELECT MIN(salary),department_idFROM employeesGROUP BY department_id#③ 在②基础上筛选,满足min(salary)>①SELECT MIN(salary),department_idFROM employeesGROUP BY department_idHAVING MIN(salary)>( SELECT MIN(salary) FROM employees WHERE department_id = 50);#非法使用标量子查询SELECT MIN(salary),department_idFROM employeesGROUP BY department_idHAVING MIN(salary)>( SELECT salary FROM employees WHERE department_id = 250);#2.列子查询(多行子查询)★#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_idFROM departmentsWHERE 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_nameFROM employeesWHERE 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 salaryFROM employeesWHERE 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,salaryFROM employeesWHERE salary'IT_PROG';#或SELECT last_name,employee_id,job_id,salaryFROM employeesWHERE 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salarySELECT last_name,employee_id,job_id,salaryFROM employeesWHERE salary 'IT_PROG';#或SELECT last_name,employee_id,job_id,salaryFROM employeesWHERE 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 employeesWHERE (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 employeesWHERE 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_idFROM employeesGROUP BY department_idSELECT * 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_salSELECT ag_dep.*,g.`grade_level`FROM ( SELECT AVG(salary) ag,department_id FROM employees GROUP BY department_id) ag_depINNER JOIN job_grades gON 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#inSELECT department_nameFROM departments dWHERE d.`department_id` IN( SELECT department_id FROM employees)#existsSELECT department_nameFROM departments dWHERE 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#inSELECT bo.*FROM boys boWHERE bo.id NOT IN( SELECT boyfriend_id FROM beauty)#existsSELECT bo.*FROM boys boWHERE NOT EXISTS( SELECT boyfriend_id FROM beauty b WHERE bo.`id`=b.`boyfriend_id`);
分页查询
#进阶8:分页查询 ★/*应用场景:当要显示的数据,一页显示不全,需要分页提交sql请求语法: select 查询列表 from 表 【join type join 表2 on 连接条件 where 筛选条件 group by 分组字段 having 分组后的筛选 order by 排序的字段】 limit 【offset,】size; offset要显示条目的起始索引(起始索引从0开始) size 要显示的条目个数特点: ①limit语句放在查询语句的最后 ②公式 要显示的页数 page,每页的条目数size select 查询列表 from 表 limit (page-1)*size,size; size=10 page 1 0 2 10 3 20 */#案例1:查询前五条员工信息SELECT * FROM employees LIMIT 0,5;SELECT * FROM employees LIMIT 5;#案例2:查询第11条——第25条SELECT * FROM employees LIMIT 10,15;#案例3:有奖金的员工信息,并且工资较高的前10名显示出来SELECT * FROM employees WHERE commission_pct IS NOT NULL ORDER BY salary DESC LIMIT 10 ;