Oracle SQL 日期之间查询

Question

我不明白为什么我的查询没有返回任何内容：

 select * from emp where hiredate between to_date ('01/01/81',
 'mm/dd/yy') and to_date ('12/31/81', 'mm/dd/yy');

该表的结构如下：

CREATE TABLE EMP ( 
EMPNO NUMBER(4) NOT NULL, 
ENAME CHAR(10), 
JOB CHAR(9), 
MGR NUMBER(4) CONSTRAINT EMP_SELF_KEY 
REFERENCES EMP (EMPNO), 
HIREDATE DATE, 
SAL NUMBER(7,2), 
COMM NUMBER(7,2), 
DEPTNO NUMBER(2) NOT NULL, 
CONSTRAINT EMP_FOREIGN_KEY FOREIGN KEY (DEPTNO) REFERENCES DEPT (DEPTNO), 
CONSTRAINT EMP_PRIM_KEY PRIMARY KEY (EMPNO)); 

INSERT INTO EMP VALUES (7839,'KING','PRESIDENT',NULL,'17-NOV-1981',5000,NULL,10); 
INSERT INTO EMP VALUES (7698,'BLAKE','MANAGER',7839,'1-MAY-1981',2850,NULL,30); 
INSERT INTO EMP VALUES (7782,'CLARK','MANAGER',7839,'9-JUN-1981',2450,NULL,10); 
INSERT INTO EMP VALUES (7566,'JONES','MANAGER',7839,'2-APR-1981',2975,NULL,20); 
INSERT INTO EMP VALUES (7654,'MARTIN','SALESMAN',7698,'28-SEP-1981',1250,1400,30); 
INSERT INTO EMP VALUES (7499,'ALLEN','SALESMAN',7698,'20-FEB-1981',1600,300,30); 
INSERT INTO EMP VALUES (7844,'TURNER','SALESMAN',7698,'8-SEP-1981',1500,0,30); 
INSERT INTO EMP VALUES (7900,'JAMES','CLERK',7698,'3-DEC-1981',950,NULL,30); 
INSERT INTO EMP VALUES (7521,'WARD','SALESMAN',7698,'22-FEB-1981',1250,500,30); 
INSERT INTO EMP VALUES (7902,'FORD','ANALYST',7566,'3-DEC-1981',3000,NULL,20); 
INSERT INTO EMP VALUES (7369,'SMITH','CLERK',7902,'17-DEC-1980',800,NULL,20); 
INSERT INTO EMP VALUES (7788,'SCOTT','ANALYST',7566,'09-DEC-1982',3000,NULL,20); 
INSERT INTO EMP VALUES (7876,'ADAMS','CLERK',7788,'12-JAN-1983',1100,NULL,20); 
INSERT INTO EMP VALUES (7934,'MILLER','CLERK',7782,'23-JAN-1982',1300,NULL,10); 

请帮助我理解。谢谢。

Answer 1

OP 中声明的另一个困难

select * from emp where hiredate between to_date ('01/01/81',
 'mm/dd/yy') and to_date ('12/31/81', 'mm/dd/yy');

是hiredate可以包含时间部分。从样本数据来看，事实并非如此，但没有什么可以阻止它发生。考虑到这一点，更好的方法（比使用更好BETWEEN）如下：

SELECT * FROM emp
 WHERE hiredate >= DATE'1981-01-01'
   AND hiredate < DATE'1982-01-01';

还可以使用TRUNC()：

SELECT * FROM emp
 WHERE TRUNC(hiredate, 'YEAR') = DATE'1981-01-01';

TO_DATE()既然 Oracle 支持 ANSI 日期文字（自 9i 起），我看不出有任何理由要使用它。