select 語句加where子句（條件限制)
1.limiting rows using a selection
select * from emp where deptno=30;
select * from emp where ename='king';
2.comparison conditions
select * from emp where sal>2500;
3.other comprison conditions
a)using between
select * from emp where sal between 900 and 2000;
b)using in
select * from emp where mgr in(7698,7788);
c)using like(%可以代表任意多個字元，而_代表任一字元）
select * from emp where ename like 'a%';
select * from emp where ename like '_a%';
d）using null
select * from emp where mgr is null;
4. logical conditions
a) using the and operator
select * from emp where sal>1000 and job like '%man%';
b) using the or operator
select * from emp where sal>1000 and job like '%man%';
c) using the not operator
select * from emp where job not in ('salesman','clerk');
5.rules of precedence （注意下面兩種情況是不同的）
select * from emp where job='salesman' or job='clerk' and sal >=1300;
select * from emp where (job='salesman' or job='clerk') and sal >=1300;
order by子句 預設的是按asc排序
1. select * from emp order by hiredate;
2. sorting in descending order
select * from emp order by hiredate desc;
3. sorting by column alias
select ename,sal,12*sal annsal from emp order by annsal;
4. sorting by multiple columns
select ename,deptno,sal annsal from emp order by deptno,sal desc;
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