【Oracle入门到删库跑路-04】基础入门：基本查询操作

零日失眠者2025-12-05 11:16

SELECT语句基础

SELECT语句是SQL中最常用的查询语句，用于从数据库中检索数据。

基本语法

sql 复制代码 
SELECT [DISTINCT] column_list
FROM table_name
[WHERE condition]
[ORDER BY column_list [ASC|DESC]]
[LIMIT number];

查询所有列

sql 复制代码 
-- 查询employees表中的所有数据
SELECT * FROM employees;

-- 查询departments表中的所有数据
SELECT * FROM departments;

查询指定列

sql 复制代码 
-- 查询员工的姓名和薪资
SELECT first_name, last_name, salary FROM employees;

-- 查询部门名称
SELECT department_name FROM departments;

使用列别名

sql 复制代码 
-- 使用AS关键字设置别名
SELECT first_name AS "名字", last_name AS "姓氏", salary AS "薪资" FROM employees;

-- 省略AS关键字
SELECT first_name "名字", last_name "姓氏", salary "薪资" FROM employees;

-- 使用表达式别名
SELECT first_name || ' ' || last_name AS "全名", salary*12 AS "年薪" FROM employees;

4.2 WHERE子句条件查询

WHERE子句用于筛选满足特定条件的记录。

比较运算符

sql 复制代码 
-- 等于
SELECT * FROM employees WHERE salary = 17000;

-- 不等于
SELECT * FROM employees WHERE department_id != 90;
SELECT * FROM employees WHERE department_id <> 90;

-- 大于、小于
SELECT * FROM employees WHERE salary > 10000;
SELECT * FROM employees WHERE hire_date < TO_DATE('2000-01-01', 'YYYY-MM-DD');

-- 大于等于、小于等于
SELECT * FROM employees WHERE salary >= 15000;
SELECT * FROM employees WHERE salary <= 10000;

逻辑运算符

sql 复制代码 
-- AND运算符
SELECT * FROM employees WHERE department_id = 60 AND salary > 8000;

-- OR运算符
SELECT * FROM employees WHERE department_id = 60 OR department_id = 90;

-- NOT运算符
SELECT * FROM employees WHERE NOT department_id = 90;

范围查询

sql 复制代码 
-- BETWEEN...AND...
SELECT * FROM employees WHERE salary BETWEEN 10000 AND 20000;

-- IN运算符
SELECT * FROM employees WHERE department_id IN (60, 90, 100);

-- NOT IN运算符
SELECT * FROM employees WHERE department_id NOT IN (60, 90, 100);

模糊查询

sql 复制代码 
-- LIKE运算符（%匹配任意字符，_匹配单个字符）
SELECT * FROM employees WHERE first_name LIKE 'A%';  -- 姓氏以A开头
SELECT * FROM employees WHERE first_name LIKE '_a%'; -- 第二个字母是a
SELECT * FROM employees WHERE first_name LIKE '____'; -- 姓氏恰好4个字符

-- NOT LIKE运算符
SELECT * FROM employees WHERE first_name NOT LIKE 'A%';

空值查询

sql 复制代码 
-- IS NULL查询空值
SELECT * FROM employees WHERE commission_pct IS NULL;

-- IS NOT NULL查询非空值
SELECT * FROM employees WHERE commission_pct IS NOT NULL;

4.3 排序查询

ORDER BY子句用于对查询结果进行排序。

单列排序

sql 复制代码 
-- 按薪资升序排列（默认）
SELECT first_name, last_name, salary FROM employees ORDER BY salary;

-- 按薪资降序排列
SELECT first_name, last_name, salary FROM employees ORDER BY salary DESC;

-- 按入职日期升序排列
SELECT first_name, last_name, hire_date FROM employees ORDER BY hire_date;

多列排序

sql 复制代码 
-- 先按部门编号升序，再按薪资降序
SELECT department_id, first_name, last_name, salary 
FROM employees 
ORDER BY department_id ASC, salary DESC;

-- 按多个字段排序
SELECT department_id, job_id, first_name, last_name, salary 
FROM employees 
ORDER BY department_id, job_id DESC, salary;

按列别名排序

sql 复制代码 
-- 按计算列排序
SELECT first_name, last_name, salary*12 AS annual_salary 
FROM employees 
ORDER BY annual_salary DESC;

4.4 函数使用

Oracle提供了丰富的内置函数用于数据处理。

字符函数

sql 复制代码 
-- UPPER函数：转换为大写
SELECT UPPER(first_name) FROM employees;

-- LOWER函数：转换为小写
SELECT LOWER(last_name) FROM employees;

-- INITCAP函数：首字母大写
SELECT INITCAP(email) FROM employees;

-- CONCAT函数：连接字符串
SELECT CONCAT(first_name, ' ') || last_name AS full_name FROM employees;

-- SUBSTR函数：截取子串
SELECT SUBSTR(first_name, 1, 3) FROM employees;

-- LENGTH函数：获取字符串长度
SELECT first_name, LENGTH(first_name) FROM employees;

-- REPLACE函数：替换字符串
SELECT REPLACE(phone_number, '.', '-') FROM employees;

数值函数

sql 复制代码 
-- ROUND函数：四舍五入
SELECT salary, ROUND(salary/7, 2) AS weekly_salary FROM employees;

-- TRUNC函数：截断
SELECT salary, TRUNC(salary/7) AS weekly_salary FROM employees;

-- MOD函数：求余数
SELECT employee_id, MOD(employee_id, 2) AS remainder FROM employees;

-- ABS函数：绝对值
SELECT ABS(-100) FROM dual;

-- CEIL函数：向上取整
SELECT CEIL(45.2) FROM dual;

-- FLOOR函数：向下取整
SELECT FLOOR(45.8) FROM dual;

日期函数

sql 复制代码 
-- SYSDATE函数：获取当前日期时间
SELECT SYSDATE FROM dual;

-- ADD_MONTHS函数：增加月份
SELECT hire_date, ADD_MONTHS(hire_date, 6) AS review_date FROM employees;

-- MONTHS_BETWEEN函数：计算月份差
SELECT first_name, hire_date, 
       ROUND(MONTHS_BETWEEN(SYSDATE, hire_date)) AS months_worked 
FROM employees;

-- NEXT_DAY函数：下一个指定星期几
SELECT NEXT_DAY(SYSDATE, 'MONDAY') FROM dual;

-- LAST_DAY函数：月末日期
SELECT LAST_DAY(SYSDATE) FROM dual;

-- EXTRACT函数：提取日期部分
SELECT EXTRACT(YEAR FROM hire_date) AS hire_year FROM employees;

转换函数

sql 复制代码 
-- TO_CHAR函数：转换为字符
SELECT TO_CHAR(SYSDATE, 'YYYY-MM-DD HH24:MI:SS') FROM dual;
SELECT first_name, TO_CHAR(salary, '$999,999.99') AS formatted_salary FROM employees;

-- TO_NUMBER函数：转换为数值
SELECT TO_NUMBER('12345') FROM dual;

-- TO_DATE函数：转换为日期
SELECT TO_DATE('2023-12-25', 'YYYY-MM-DD') FROM dual;

4.5 分组查询

GROUP BY子句用于将查询结果按指定列分组。

基本分组

sql 复制代码 
-- 按部门分组统计员工数量
SELECT department_id, COUNT(*) AS employee_count 
FROM employees 
GROUP BY department_id;

-- 按部门分组统计平均薪资
SELECT department_id, AVG(salary) AS avg_salary 
FROM employees 
GROUP BY department_id;

多列分组

sql 复制代码 
-- 按部门和职位分组
SELECT department_id, job_id, COUNT(*) AS employee_count, AVG(salary) AS avg_salary
FROM employees 
GROUP BY department_id, job_id
ORDER BY department_id, job_id;

HAVING子句

HAVING子句用于对分组后的结果进行筛选。

sql 复制代码 
-- 查询员工数量大于5的部门
SELECT department_id, COUNT(*) AS employee_count 
FROM employees 
GROUP BY department_id 
HAVING COUNT(*) > 5;

-- 查询平均薪资大于10000的部门
SELECT department_id, AVG(salary) AS avg_salary 
FROM employees 
GROUP BY department_id 
HAVING AVG(salary) > 10000;

聚合函数

sql 复制代码 
-- COUNT函数：统计记录数
SELECT COUNT(*) FROM employees;
SELECT COUNT(commission_pct) FROM employees;  -- 不统计NULL值

-- SUM函数：求和
SELECT SUM(salary) AS total_salary FROM employees;

-- AVG函数：平均值
SELECT AVG(salary) AS avg_salary FROM employees;

-- MAX函数：最大值
SELECT MAX(salary) AS max_salary FROM employees;

-- MIN函数：最小值
SELECT MIN(salary) AS min_salary FROM employees;

4.6 多表连接查询

连接查询用于从多个相关表中检索数据。

内连接（INNER JOIN）

sql 复制代码 
-- 显示员工姓名和所属部门名称
SELECT e.first_name, e.last_name, d.department_name
FROM employees e
INNER JOIN departments d ON e.department_id = d.department_id;

-- 使用传统语法
SELECT e.first_name, e.last_name, d.department_name
FROM employees e, departments d
WHERE e.department_id = d.department_id;

外连接

sql 复制代码 
-- 左外连接（LEFT JOIN）
SELECT e.first_name, e.last_name, d.department_name
FROM employees e
LEFT JOIN departments d ON e.department_id = d.department_id;

-- 右外连接（RIGHT JOIN）
SELECT e.first_name, e.last_name, d.department_name
FROM employees e
RIGHT JOIN departments d ON e.department_id = d.department_id;

-- 全外连接（FULL JOIN）
SELECT e.first_name, e.last_name, d.department_name
FROM employees e
FULL JOIN departments d ON e.department_id = d.department_id;

自连接

sql 复制代码 
-- 查询员工及其管理者信息
SELECT e.first_name || ' ' || e.last_name AS employee,
       m.first_name || ' ' || m.last_name AS manager
FROM employees e
LEFT JOIN employees m ON e.manager_id = m.employee_id;

多表连接

sql 复制代码 
-- 连接三个表：员工、部门、工作地点
SELECT e.first_name, e.last_name, d.department_name, l.city
FROM employees e
JOIN departments d ON e.department_id = d.department_id
JOIN locations l ON d.location_id = l.location_id;

4.7 子查询

子查询是嵌套在其他查询中的查询语句。

单行子查询

sql 复制代码 
-- 查询薪资高于平均薪资的员工
SELECT first_name, last_name, salary
FROM employees
WHERE salary > (SELECT AVG(salary) FROM employees);

-- 查询与员工100同部门的员工
SELECT first_name, last_name, department_id
FROM employees
WHERE department_id = (SELECT department_id FROM employees WHERE employee_id = 100);

多行子查询

sql 复制代码 
-- 使用IN运算符
SELECT first_name, last_name, department_id
FROM employees
WHERE department_id IN (SELECT department_id FROM departments WHERE location_id = 1700);

-- 使用ANY运算符
SELECT first_name, last_name, salary
FROM employees
WHERE salary > ANY (SELECT salary FROM employees WHERE department_id = 60);

-- 使用ALL运算符
SELECT first_name, last_name, salary
FROM employees
WHERE salary > ALL (SELECT salary FROM employees WHERE department_id = 60);

相关子查询

sql 复制代码 
-- 查询每个部门薪资最高的员工
SELECT department_id, first_name, last_name, salary
FROM employees e1
WHERE salary = (SELECT MAX(salary) 
                FROM employees e2 
                WHERE e2.department_id = e1.department_id);

4.8 集合操作

集合操作用于合并多个查询结果。

UNION操作

sql 复制代码 
-- 合并两个查询结果（去除重复）
SELECT first_name, last_name FROM employees WHERE department_id = 60
UNION
SELECT first_name, last_name FROM employees WHERE department_id = 90;

UNION ALL操作

sql 复制代码 
-- 合并两个查询结果（保留重复）
SELECT first_name, last_name FROM employees WHERE department_id = 60
UNION ALL
SELECT first_name, last_name FROM employees WHERE department_id = 90;

INTERSECT操作

sql 复制代码 
-- 返回两个查询结果的交集
SELECT department_id FROM departments WHERE location_id = 1700
INTERSECT
SELECT department_id FROM employees WHERE salary > 10000;

MINUS操作

sql 复制代码 
-- 返回第一个查询结果中存在但第二个查询结果中不存在的记录
SELECT department_id FROM departments
MINUS
SELECT department_id FROM employees;

4.9 本章小结

本章详细介绍了Oracle数据库中的基本查询操作，包括SELECT语句的使用、WHERE条件筛选、排序、函数使用、分组查询、多表连接、子查询和集合操作等。掌握这些查询技能是进行数据分析和报表生成的基础。

练习题

  1. 查询所有员工的姓名、邮箱和薪资，并按薪资降序排列
  2. 查询薪资在10000到20000之间的员工信息
  3. 查询姓氏以'S'开头的员工
  4. 查询没有佣金的员工数量
  5. 按部门统计员工数量和平均薪资
  6. 查询每个部门薪资最高的员工信息
  7. 查询员工姓名及其管理者姓名
  8. 使用集合操作查询在部门60或部门90工作但不在部门100工作的员工
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