SQL 基础-- 子查询
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--SQL 基礎(chǔ)--> 子查詢
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/*
一、子查詢
??? 子查詢就是位于SELECT、UPDATE、或DELETE語(yǔ)句中內(nèi)部的查詢
???
二、子查詢的分類
??? 單行子查詢
??????? 返回零行或一行
??? 多行子查詢
??????? 返回一行或多行
??? 多列子查詢
??????? 返回多列
??? 相關(guān)子查詢
??????? 引用外部SQL語(yǔ)句中的一列或多列
??? 嵌套子查詢
??????? 位于其它子查詢中的查詢
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三、子查詢語(yǔ)法 */
??? SELECT select_list
??? FROM table
??? WHERE expr operator
?????? (SELECT select_list
?????? ?FROM table);
?????? ?/*
??? 子查詢(內(nèi)部查詢)在執(zhí)行主查詢之前執(zhí)行一次
??? 然后主查詢(外部查詢)會(huì)使用該子查詢的結(jié)果?
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四、子查詢的規(guī)則
??? 將子查詢括在括號(hào)中
??? 將子查詢放置在比較條件的右側(cè)
??? 只有在執(zhí)行排序Top-N分析時(shí)，子查詢中才需要使用ORDER BY 子句
??? 單行運(yùn)算符用于單行子查詢，多行運(yùn)算符用于多行子查詢?
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五、單行子查詢
??? 僅返回一行
??? 使用單行的表較運(yùn)算符：= ,>, >= ,< , <= ,<>
???
??? 在WHERE 子句中使用子查詢??? */
??? SQL> select ename,job from emp
??? ? 2? where empno = (
??? ? 3??? select empno from emp
??? ? 4??? where mgr = 7902 );
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??? ENAME????? JOB
??? ---------- ---------
??? SMITH????? CLERK
???
??? --使用分組函數(shù)的子查詢
??? SQL> select ename,job,sal
??? ? 2? from emp
??? ? 3? where sal >
??? ? 4??? (select avg(sal) from emp);
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??? ENAME????? JOB????????????? SAL
??? ---------- --------- ----------
??? JONES????? MANAGER???????? 2975
??? BLAKE????? MANAGER???????? 2850
??? CLARK????? MANAGER???????? 2450
??? SCOTT????? ANALYST???????? 3000
??? KING?????? PRESIDENT?????? 5000
??? FORD?????? ANALYST???????? 3000
???
??? --在HAVING子句中使用子查詢
??? SQL> select deptno,min(sal)
??? ? 2? from emp
??? ? 3? group by deptno
??? ? 4 ?having min(sal) >
??? ? 5????? (select min(sal)
??? ? 6?????? from emp
??? ? 7?????? where deptno = 20);
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?????? DEPTNO?? MIN(SAL)
??? ---------- ----------
?????????? 30??????? 950
?????????? 10?????? 1300
??????????
??? --在FROM 子句中使用子查詢
??? SQL> select empno,ename
??? ? 2? from
??? ? 3????? (select empno,ename
??? ? 4?????? from emp
??? ? 5?????? where deptno = 20);
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?????? ?EMPNO ENAME
??? ---------- ----------
?????? ? 7369 SMITH
?????? ? 7566 JONES
?????? ? 7788 SCOTT
?????? ? 7876 ADAMS
?????? ? 7902 FORD
???
??? --單行子查詢中的常見(jiàn)錯(cuò)誤
??? ??? --子查詢的結(jié)果返回多于一行
?????? SQL> select empno,ename
?????? ? 2? from emp
?????? ? 3? where sal =
?????? ? 4????? (select sal?????
?????? ? 5?????? from emp
?????? ? 6?????? where deptno = 20);
?????????? (select sal
?????????? ?*
?????? ERROR at line 4:
?????? ORA-01427: single-row subquery returns more than one row
??????
?????? --子查詢中不能包含ORDER BY子句
?????? SQL> select empno,ename
?????? ? 2? from emp
?????? ? 3? where sal >
?????? ? 4????? (select avg(sal)
?????? ? 5?????? from emp
?????? ? 6?????? order by empno);
?????????? ?order by empno)
?????????? ?*
?????? ERROR at line 6:
?????? ORA-00907: missing right parenthesis
??????
?????? --子查詢內(nèi)部沒(méi)有返回行，如下語(yǔ)句可以正確執(zhí)行，但沒(méi)有數(shù)據(jù)返回
?????? SQL> select ename,job
?????? ? 2? from emp
?????? ? 3? where empno =
?????? ? 4????? (select empno
?????? ? 5?????? from emp
?????? ? 6?????? where mgr = 8000);
?
?????? no rows selected
?
??? /*?
六、多行子查詢
??? 返回多個(gè)行
??? 使用多行比較運(yùn)算符IN ,ANY ,ALL
???
??? 在多行子查詢中使用IN 操作符? */
??? SQL> select empno,ename,job
??? ? 2? from emp
??? ? 3? where sal in
??? ? 4????? (select max(sal)
??? ? 5?????? from emp
??? ? 6?????? group by deptno);
?
?????? ?EMPNO ENAME????? JOB
??? ---------- ---------- ---------
?????? ? 7698 BLAKE????? MANAGER
?????? ? 7902 FORD?????? ANALYST
?????? ? 7788 SCOTT????? ANALYST
?????? ? 7839 KING?????? PRESIDENT
?????? ?
??? --在多行子查詢中使用ANY 操作符
??? SQL> select empno,ename,job
??? ? 2? from emp
??? ? 3? where sal < any
??? ? 4????? (select avg(sal)
??? ? 5?????? from emp
??? ? 6?????? group by deptno);
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?????? ?EMPNO ENAME????? JOB
??? ---------- ---------- ---------
??? ??? ? 7369 SMITH????? CLERK
?????? ? 7900 JAMES????? CLERK
?????? ? 7876 ADAMS????? CLERK
?????? ? 7521 WARD?????? SALESMAN
?????? ? 7654 MARTIN???? SALESMAN
?????? ? 7934 MILLER???? CLERK
?????? ? 7844 TURNER???? SALESMAN
?????? ? 7499 ALLEN????? SALESMAN
?????? ? 7782 CLARK????? MANAGER
?????? ? 7698 BLAKE????? MANAGER
?????? ?
??? --在多行子查詢中使用ALL 操作符
??? SQL> select empno,ename,job
??? ? 2? from emp
??? ? 3? where sal > all
??? ? 4????? (select avg(sal)
??? ? 5?????? from emp
??? ? 6*????? group by deptno)
???
?????? ?EMPNO ENAME????? JOB
??? ---------- ---------- ---------
?????? ? 7566 JONES????? MANAGER
?????? ? 7788 SCOTT????? ANALYST
?????? ? 7839 KING?????? PRESIDENT
?????? ? 7902 FORD?????? ANALYST
???
???? /*
七、相關(guān)子查詢
??? 子查詢中使用了主查詢中的某些字段，主查詢每掃描一行都要執(zhí)行一次子查詢 */
?
??? --查詢工資高于同一部門的員工的部門號(hào)，姓名，工資
??? SQL> select deptno,ename,sal
??? ? 2? from emp outer
??? ? 3? where sal >
??? ? 4????? (select avg(sal)
??? ? 5?????? from emp inner
??? ? 6?????? where inner.deptno = outer.deptno);
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?????? DEPTNO ENAME???????????? SAL
??? ---------- ---------- ----------
?????????? 30 ALLEN??????????? 1600
?????????? 20 JONES??????????? 2975
?????????? 30 BLAKE??????????? 2850
?????????? 20 SCOTT??????????? 3000
?????????? 10 KING???????????? 5000
?????????? 20 FORD???????????? 3000
??????????
??? --查詢負(fù)責(zé)管理其它員工的員工記錄(使用exists)
??? SQL> select empno,ename
??? ? 2? from emp outer
??? ? 3? where exists
??? ? 4???? (select empno
??? ? 5????? from emp inner
??? ? 6????? where inner.mgr = outer.empno);
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?????? ?EMPNO ENAME
??? ---------- ----------
?????? ? 7566 JONES
?????? ? 7698 BLAKE
?????? ? 7782 CLARK
?????? ? 7788 SCOTT
?????? ? 7839 KING
?????? ? 7902 FORD
??? --查詢不管理其它員工的職員(not exists)
??? SQL> l3
??? ? 3* where exists
??? SQL> c /where/where not
??? ? 3* where not exists
??? SQL> l
??? ? 1? select empno,ename
??? ? 2? from emp outer
??? ? 3? where not exists
??? ? 4???? (select empno
??? ? 5????? from emp inner
??? ? 6*???? where inner.mgr = outer.empno)
??? SQL> /
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?????? ?EMPNO ENAME
??? ---------- ----------
?????? ? 7369 SMITH
?????? ? 7499 ALLEN
?????? ? 7521 WARD
?????? ? 7654 MARTIN
?????? ? 7844 TURNER
?????? ? 7876 ADAMS
?????? ? 7900 JAMES
?????? ? 7934 MILLER
?????? ?
??? EXISTS 和NOT EXISTS 與IN 和NOT IN 的比較
??????? EXISTS與IN的不同：
??????????? EXISTS只檢查行的存在性,IN 要檢查實(shí)際值的存在性(一般情況下EXISTS的性能高于IN)
??? ??? NOT EXISTS 和NOT IN
?????? ??? 當(dāng)值列表中包含空值的情況下,NOT EXISTS 則返回true,而NOT IN 則返回false.
??????????
??? --看下面的查詢，查詢部門號(hào)不在emp表中出現(xiàn)的部門名稱及位置
??? SQL> select deptno,dname,loc
??? ? 2? from dept d
??? ? 3? where not exists
??? ? 4????? (select 1
??? ? 5?????? from emp e
??? ? 6*????? where e.deptno = d.deptno)
??? ?
?????? DEPTNO DNAME????????? LOC
??? ---------- -------------- -------------
?????????? 40 OPERATIONS???? BOSTON
??? ??
??? --IN與空值
?????????? SQL> SELECT *
???????????? 2??? FROM emp e
???????????? 3??? WHERE e.empno NOT IN (
???????????? 4?????????????????????????? SELECT 7369 FROM dual
???????????? 5?????????????????????????? UNION ALL
???????????? 6?????????????????????????? SELECT NULL FROM dual
???????????? 7????????????????????????? )
???????????? 8? ;
???????????
?????????? EMPNO ENAME????? JOB???????? MGR HIREDATE ?????????SAL????? COMM DEPTNO
?????????? ----- ---------- --------- ----- ----------- --------- --------- ------
???????????
?????????? SQL> SELECT *
???????????? 2??? FROM emp e
???????????? 3??? WHERE e.empno IN ('7369',NULL)
???????????? 4? ;
???????????
?????????? EMPNO ENAME????? JOB???????? MGR HIREDATE????????? SAL????? COMM DEPTNO
?????????? ----- ---------- --------- ----- ----------- --------- --------- ------
??????????? 7369 SMITH????? CLERK????? 7902 1980-12-17???? 800.00?????????????? 20
???????????
????
??? /*?
? ? 注：子查詢要包含在括號(hào)內(nèi)
??? ??? 子查詢一般放在比較條件的右側(cè)
??? ??? 除非進(jìn)行TOP－N 分析，否則不要在子查詢中使用ORDER BY。 */
?
??? /*
八、多列子查詢
??? 1、成對(duì)比較
??? 查詢工資為部門最高的記錄 */
??? SQL> select * from scott.emp
??? ? 2? where (sal,job) in
??? ? 3???? (select max(sal),job from scott.emp group by job);
???
?????? ?EMPNO ENAME????? JOB????????????? MGR HIREDATE???????? SAL?????? COMM???? DEPTNO
??? ---------- ---------- --------- ---------- --------- ---------- ---------- ----------
?????? ? 7934 MILLER???? CLERK?????????? 7782 23-JAN-82?????? 1300??????????????????? 10
?????? ? 7499 ALLEN????? SALESMAN??????? 7698 20-FEB-81?????? 1600??????? 300???????? 30
?????? ? 7839 KING?????? PRESIDENT??????????? 17-NOV-81?????? 5000??????????????????? 10
?????? ? 7566 JONES????? MANAGER???????? 7839 02-APR-81?????? 2975??????????????????? 20
?????? ? 7902 FORD?????? ANALYST???????? 7566 03-DEC-81?????? 3000??????????????????? 20
?????? ? 7788 SCOTT????? ANALYST???????? 7566 19-APR-87?????? 3000??????????????????? 20???
??????
?????? /*??????
??? 2、非成對(duì)比較,實(shí)現(xiàn)了與上述類似的功能*/
??? SQL> select * from scott.emp
??? ? 2? where sal in (select max(sal) from scott.emp group by job)
??? ? 3? and job in (select distinct job from scott.emp);
?
?????? ?EMPNO ENAME????? JOB????????????? MGR HIREDATE???????? SAL?????? COMM???? DEPTNO
??? ---------- ---------- --------- ---------- --------- ---------- ---------- ----------
?????? ? 7934 MILLER???? CLERK?????????? 7782 23-JAN-82?????? 1300??????????????????? 10
?????? ? 7499 ALLEN????? SALESMAN??????? 7698 20-FEB-81?????? 1600??????? 300???????? 30
?????? ? 7566 JONES????? MANAGER???????? 7839 02-APR-81?????? 2975??????????????????? 20
?????? ? 7788 SCOTT????? ANALYST???????? 7566 19-APR-87?????? 3000??????????????????? 20
?????? ? 7902 FORD?????? ANALYST???????? 7566 03-DEC-81?????? 3000??????????????????? 20
?????? ? 7839 KING?????? PRESIDENT??????????? 17-NOV-81?????? 5000??????????????????? 10
?????? ?
???? /*
九、嵌套子查詢
??? 即位于子查詢內(nèi)部的子查詢，嵌套層數(shù)最多可達(dá)層。然而應(yīng)盡量避免使用嵌套子查詢，使用表連接的查詢性能會(huì)更高*/
??? SQL> select deptno,Num_emp
??? ? 2? from (select deptno,count(empno) as Num_emp from emp group by deptno) d
??? ? 3? where Num_emp > 3;
?
?????? DEPTNO??? NUM_EMP
??? ---------- ----------
?????????? 30????????? 6
?????????? 20????????? 5
??????
??? /*?????
??? 注意：子查詢對(duì)空值的處理
??? 除了count(*)外，都會(huì)忽略掉空值 */
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??? /*
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