一、使用WHERE 子句，将不满足条件的行过滤掉。
WHERE 子句紧随 FROM 子句
eg:
SELECT employee_id, last_name, job_id, department_id
FROM employees
WHERE department_id = 90
字符和日期
字符和日期要包含在单引号中。
字符大小写敏感，日期格式敏感。
默认的日期格式是 DD-MON月-RR。
eg1:
SELECT last_name, job_id, department_id
FROM employees
WHERE last_name = 'Whalen';
eg2:
SELECT last_name, hire_date, department_id
FROM employees
WHERE hire_date = '7-6月-1994'
2、比较运算
eg:
SELECT last_name, salary
FROM employees
WHERE salary <= 3000;
3、其它比较运算
使用 BETWEEN 运算来显示在一个区间内的值
eg:
SELECT last_name, salary
FROM employees
WHERE salary BETWEEN 2500 AND 3500;
使用 IN运算显示列表中的值。
eg:
SELECT employee_id, last_name, salary, manager_id
FROM employees
WHERE manager_id IN (100, 101, 201);
LIKE
使用 LIKE 运算选择类似的值
选择条件可以包含字符或数字:
% 代表零个或多个字符(任意个字符)。
_ 代表一个字符。
eg:
SELECTfirst_name
FROM employees
WHEREfirst_name LIKE 'S%';
‘%’和‘-’可以同时使用。
eg:
SELECT last_name
FROM employees
WHERE last_name LIKE '_o%';
可以使用 ESCAPE 标识符 选择‘%’和 ‘_’ 符号。
回避特殊符号的：使用转义符。例如：将[%]转为[\%]、[_]转为[\_]，然后再加上[ESCAPE ‘\’] 即可
eg:
SELECT job_id
FROM jobs
WHERE job_id LIKE ‘IT\_%‘ escape ‘\‘;
NULL
使用 IS (NOT) NULL 判断空值。
eg:
ELECT last_name, manager_id
FROM employees
WHERE manager_id IS NULL;
4、逻辑运算
AND 逻辑并
OR 逻辑或
NOT 逻辑否
AND 要求并的关系为真。
eg:
SELECT employee_id, last_name, job_id, salary
FROM employees
WHERE salary >=10000
AND job_id LIKE '%MAN%';
OR 要求或关系为真。
eg:
SELECT employee_id, last_name, job_id, salary
FROM employees
WHERE salary >= 10000
OR job_id LIKE '%MAN%';
NOT
eg:
SELECT last_name, job_id
FROM employees
WHERE job_id
NOT IN ('IT_PROG', 'ST_CLERK', 'SA_REP');
优先级
二、排序
ORDER BY子句
使用 ORDER BY 子句排序
ASC（ascend）: 升序
DESC（descend）: 降序
ORDER BY 子句在SELECT语句的结尾。
eg1:SELECT last_name, job_id, department_id, hire_date
FROM employees
ORDER BY hire_date ;
eg2:
SELECT last_name, job_id, department_id, hire_date
FROM employees
ORDER BY hire_date DESC ;
按别名排序
eg:
SELECT employee_id, last_name, salary*12 annsal
FROM employees
ORDER BY annsal;
多个列排序
eg:
SELECT last_name, department_id, salary
FROM employees
ORDER BY department_id, salary DESC;
总 结
使用WHERE 子句过滤数据
使用比较运算
使用 BETWEEN AND, IN, LIKE和 NULL运算
使用逻辑运算符 AND, OR和NOT
使用 ORDER BY 子句进行排序。