SQL，如何利用标准编程语言高效管理关系数据库？

1、SELECT: 用于从数据库表中检索数据。

“`sql

SELECT column1, column2, … FROM table_name;

“`

要从名为 "employees" 的表中选择所有员工的姓名和工资：

“`sql

SELECT name, salary FROM employees;

“`

2、WHERE: 用于过滤结果集，只返回满足指定条件的记录。

“`sql

SELECT column1, column2, … FROM table_name WHERE condition;

“`

要选择年龄大于30岁的员工的名字和工资：

“`sql

SELECT name, salary FROM employees WHERE age > 30;

“`

3、ORDER BY: 用于对结果集进行排序。

“`sql

SELECT column1, column2, … FROM table_name ORDER BY column_name ASC|DESC;

“`

按工资降序排列员工：

“`sql

SELECT name, salary FROM employees ORDER BY salary DESC;

“`

4、GROUP BY: 用于将结果集按照一个或多个列进行分组。

“`sql

SELECT column1, aggregate_function(column2) FROM table_name GROUP BY column1;

“`

计算每个部门的平均薪水：

“`sql

SELECT department, AVG(salary) FROM employees GROUP BY department;

“`

5、HAVING: 用于过滤聚合函数的结果。

“`sql

SELECT column1, aggregate_function(column2) FROM table_name GROUP BY column1 HAVING condition;

“`

找出平均薪水超过50000的部门：

“`sql

SELECT department, AVG(salary) FROM employees GROUP BY department HAVING AVG(salary) > 50000;

“`

6、JOIN: 用于结合两个或多个表的行。

“`sql

SELECT column1, column2, … FROM table1 JOIN table2 ON table1.column = table2.column;

“`

列出所有员工及其所属部门的名称：

“`sql

SELECT employees.name, departments.department_name FROM employees JOIN departments ON employees.department_id = departments.department_id;

“`

7、INSERT INTO: 用于向表中插入新记录。

“`sql

INSERT INTO table_name (column1, column2, …) VALUES (value1, value2, …);

“`

添加一个新员工：

“`sql

INSERT INTO employees (name, salary, age) VALUES (‘John Doe’, 60000, 35);

“`

8、UPDATE: 用于修改表中的现有记录。

“`sql

UPDATE table_name SET column1 = value1, column2 = value2, … WHERE condition;

“`

将所有年龄大于40的员工的工资增加10%：

“`sql

UPDATE employees SET salary = salary * 1.1 WHERE age > 40;

“`

9、DELETE: 用于从表中删除记录。

“`sql

DELETE FROM table_name WHERE condition;

“`

删除所有年龄小于25岁的员工：

“`sql

DELETE FROM employees WHERE age < 25;

“`

10、LIMIT: 用于限制返回的记录数。

“`sql

SELECT column1, column2, … FROM table_name LIMIT number;

“`

仅显示前5名薪水最高的员工：

“`sql

SELECT name, salary FROM employees ORDER BY salary DESC LIMIT 5;

“`

相关问题与解答：

1、问题: 如何在 SQL 中使用子查询？

解答: 在 SQL 中，可以在另一个 SQL 查询内部使用子查询，子查询可以出现在各种 SQL 语句中，如 SELECT、FROM、WHERE 和 HAVING 子句，子查询必须始终用括号括起来，并且通常放在比较运算符（如 =、<>、>、<、>=、<=）之后，要找到工资高于平均工资的员工，可以使用以下子查询：

“`sql

SELECT name, salary FROM employees WHERE salary > (SELECT AVG(salary) FROM employees);

“`