pg14-sql基础（二）-排序与统计

排序

SELECT employee_id, first_name, last_name, hire_date, salary
FROM employees
ORDER BY first_name; --按字母，默认升序
ORDER BY hire_date ASC; --升序
ORDER BY hire_date DESC; --降序
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SELECT employee_id, first_name, last_name, hire_date, salary
FROM employees
ORDER BY first_name, last_name DESC --姓按照升序排，名按照降序排
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SELECT employee_id, first_name, last_name, hire_date, salary
FROM employees
ORDER BY 2, 3 DESC --也可输入要查询字段的顺序号码
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SELECT employee_id, manager_id
FROM employees
ORDER BY manager_id DESC; --排序时null是最大的值

SELECT employee_id, manager_id
FROM employees
ORDER BY manager_id NULLS FIRST; --指定排序时null是第一
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返回top

-- 前十名
SELECT first_name, last_name, salary
FROM employees
ORDER BY salary
-- FETCH FIRST 10 ROWS ONLY;
LIMIT 10; --无相同值
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SELECT first_name, last_name, salary
FROM employees
ORDER BY salary
FETCH FIRST 10 ROWS WITH TIES;
-- 返回数据包含相同值
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SELECT first_name, last_name, salary
FROM employees
ORDER BY salary
OFFSET 10 ROWS --实现翻页，第一次1-10，第二次11-20，第三次，21-30
-- OFFSET 20 ROWS
-- OFFSET 30 ROWS
FETCH FIRST 10 ROWS ONLY;
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SELECT first_name, last_name, salary
FROM employees
ORDER BY salary
LIMIT 10 OFFSET 10; --同实现翻页
-- LIMIT 10 OFFSET 20;
-- LIMIT 10 OFFSET 20;
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条件表达case

SELECT first_name, last_name,
  CASE
     WHEN salary <5000 THEN '低收入'
	  WHEN salary between 5000 and 100000 THEN '中等收入'
	  ELSE '高收入'
  END AS "收入等级"
FROM employees;
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--查询部门编号为10、20、30有各多少人
SELECT COUNT(CASE department_id WHEN 10 THEN 1 END) dep_cnt_10,
-- 换种写法：COUNT(*) FILTER (WHERE department_id=10) dep_cnt_10,
       COUNT(CASE department_id WHEN 20 THEN 1 END) dep_cnt_20,
	   COUNT(CASE department_id WHEN 30 THEN 1 END) dep_cnt_30
FROM employees;
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