sql语句练习，oracle与mysql不同

sql语句，oracle与mysql不同

两表联查练习
– 查询财务部女员工的信息

SELECT
	*
FROM
	t_emp e
LEFT JOIN t_dept d ON e.dept_id = d.dept_id
WHERE
	d.dept_name = '财务部'
AND e.emp_sex = 0

– 查询财务部工资大于2000的男员工的信息

SELECT
	*
FROM
	t_emp e
LEFT JOIN t_dept d ON e.dept_id = d.dept_id
WHERE
	d.dept_name = '财务部'
AND e.emp_sex = 1
AND e.emp_wage > 2000

order by的用法
– 查询所有员工的姓名、工资、部门名称，并以工资倒序排列

SELECT
	e.emp_name,
	e.emp_wage,
	d.dept_name
FROM
	t_emp e
LEFT JOIN t_dept d ON e.dept_id = d.dept_id
ORDER BY
	e.emp_wage DESC

– 查出工资大于5000的员工id、员工姓名、部门名称、工资

SELECT
	e.emp_id,
	e.emp_name,
	e.emp_wage,
	d.dept_name
FROM
	t_emp e
LEFT JOIN t_dept d ON e.dept_id = d.dept_id
WHERE
	e.emp_wage > 5000

– 查询年龄大于20的李姓男员工的员工姓名、部门名称

SELECT
	e.emp_name,
	d.dept_name
FROM
	t_emp e
LEFT JOIN t_dept d ON e.dept_id = d.dept_id
WHERE
	e.emp_age > 20
AND e.emp_sex = 1
AND e.emp_name LIKE '李%'

聚合函数max的用法
– 查询工资大于男员工最大工资的女员工的员工id、员工姓名、部门名称

SELECT
	e.emp_id,
	e.emp_name,
	d.dept_name
FROM
	t_emp e
LEFT JOIN t_dept d ON e.dept_id = d.dept_id
WHERE
	e.emp_wage > (
		SELECT
			max(emp_wage)
		FROM
			t_emp
		WHERE
			emp_sex = 1
	)
AND e.emp_sex = 0

group by的用法
– 查询至少有三个员工的部门名称

select 
d.dept_name
FROM
	t_emp e
LEFT JOIN t_dept d ON e.dept_id = d.dept_id
GROUP BY e.dept_id having count(e.emp_id)>=3

聚合函数与group by共同使用语法：group by后边不能跟where语句，只能用having
– 查询最大工资大于5000的部门名称

SELECT
	d.dept_name
FROM
	t_emp e
LEFT JOIN t_dept d ON e.dept_id = d.dept_id
GROUP BY e.dept_id HAVING MAX(e.emp_wage) > 5000

– 查询工资最高的张姓女员工的员工id和部门名称

SELECT
	e.emp_id,
	d.dept_name
FROM
	t_emp e
LEFT JOIN t_dept d ON e.dept_id = d.dept_id
WHERE
	e.emp_wage = (
		SELECT
			max(emp_wage)
		FROM
			t_emp
		WHERE
			emp_name LIKE '张%' and emp_sex = 0
	)
AND 	emp_name LIKE '张%' and emp_sex = 0

**

分页：oracle与mysql的区别

**

– 分页查询员工表的信息，假设每页展示5条，查询第3页的数据
– oracle

SELECT
	*
FROM
	(
		SELECT
			rownum r,
			e.*
		FROM
			(SELECT * FROM t_emp) e
		WHERE
			rownum <= 3 * 5
	)
WHERE
	r > (3 - 1) * 5

– mysql
– mysql识别不了后边进行运算，需要计算好了在拿出来

select * from t_emp LIMIT 4 , 2;
//错误的
select * from t_emp LIMIT (3-1)*2 , 2;
select * from  t_emp limit  (3-1)*5 , 5 ;

– 将财务部工资大于2000的员工的工资在原有基础上减100

UPDATE t_emp
SET emp_wage = emp_wage + 100
WHERE
	emp_wage > 2000
AND dept_id = (
	SELECT
		dept_id
	FROM
		t_dept
	WHERE
		dept_name = '财务部'
)

– 将财务部年龄最大的员工的工资改为20000

– oracle

UPDATE t_emp
SET emp_wage = 20000
WHERE
	emp_age = (
		SELECT
			max(emp_age)
		FROM
			t_emp
		WHERE
			dept_id = (
				SELECT
					dept_id
				FROM
					t_dept
				WHERE
					dept_name = '财务部'
			)
	)

– mysql，对于一些语句执行不了，会报错，就比如这个，它识别不了oracle这种，必须也要把后边的条件当作一张表才可以继续执行

UPDATE t_emp
SET emp_wage = 20000
WHERE
	emp_age = (
		SELECT
			*
		FROM
			(
				SELECT
					max(emp_age)
				FROM
					t_emp
				WHERE
					dept_id = (
						SELECT
							dept_id
						FROM
							t_dept
						WHERE
							dept_name = '财务部'
					)
			) AS emp
	)

– 删除员工表的重复信息，认为姓名和部门一样是重复的

– oracle

DELETE
FROM
	t_emp
WHERE
	emp_id NOT IN (
		SELECT
			emp_id
		FROM
			t_emp
		GROUP BY
			emp_name,
			dept_id
	);

– mysql

DELETE
FROM
	t_emp
WHERE
	emp_id NOT IN (
		SELECT
			*
		FROM
			(
				SELECT
					emp_id
				FROM
					t_emp
				GROUP BY
					emp_name,
					dept_id
			) AS emp
	);