组函数和表关系，
第三章组函数和表关系
字符串函数
-- concat 连接两个字符串
select concat('abc','ABC') from dual;
select 'abc'||'ABC' from dual;
select concat(first_name,'_')||last_name con,t.* from EMPLOYEES t;
-- initcap 返回字符串，第一个大写，其余小写
select initcap(email) from EMPLOYEES;
-- length 返回字符串长度
select length(email),t.* from EMPLOYEES t;
-- lower 所有字符小写
-- upper 所有字符大写
select lower(first_name),upper(last_name),t.* from EMPLOYEES t;
-- substr 字符串截取
select substr('123456789',3,4) from dual;
从第 3个 截取，截取 4 个字符
-- replace 字符串替换
select replace('he love you', 'he', 'i') from dual;
I love you
数学函数
-- CEIL 返回大于或等于给出数字的最大整数
select ceil('123.456789') from dual;
select ceil('123') from dual;
-- FLOOR 返回小于或等于给出数字的最小整数
select floor('123.456789') from dual;
-- Round 函数进行四舍五入
select round(124.1666,-2),round(124.1666,2) from dual;
-2,小数点前两位，四舍五入
2，小数点前两位，四舍五入
-- trunc 函数进行截取，直接截取，不进行四舍五入
select trunc(124.1666,-2) , trunc(124.16666,2) from dual;
日期函数
select sysdate from dual;
select to_char(sysdate,'yyyy-mm-dd hh24:mi:ss') from dual;
select to_char(sysdate,'yyyy-mm-dd hh12:mi:ss') from dual;
select to_date('2019-02-27 18:30:31','yyyy-mm-dd hh24:mi:ss') from dual;
select systimestamp from dual;
select to_timestamp('2019-02-27 18:30:31.123','yyyy-mm-dd hh24:mi:ss.ff') from dual;
-- add_Months 增加或减去月份
select add_months(sysdate，2) from dual;
select add_months(sysdate，-2) from dual;
-- months_between 显示日期相差的月数
select months_between(
to_date('2019-02-01','yyyy-mm-dd')，
to_date('2019-10-01','yyyy-mm-dd')
) from dual;
聚合函数，组函数
-- max 最大值
select department_id,max(salary) from EMPLOYEES group by department_id;
-- min 最小值
select manager_id,min(salary) from EMPLOYEES group by manager_id;
-- avg 平均
SELECT department_id,AVG(salary) AVGSAL FROM EMPLOYEES GROUP BY department_id;
-- sum 总和
SELECT department_id,SUM(salary) SUMSAL FROM EMPLOYEES GROUP BY department_id;
-- count 统计
select count(1) from EMPLOYEES;
表关系 设计阶段
多表连接 SQL实现阶段
示例：
