SQL

Question

我有一个 employees table 和一个 hire_date 列，我想做的查询是计算 1995 年和 1996 年雇用了多少员工。1995 年在哪里有自己的专栏，1996 有自己的专栏：

 ______________
|      |      |
| 1995 | 1996 | 
|______|______|
|   2  |   3  | 
|______|______|`

伪代码：

display COUNT of rows WHERE hire_date LIKE '%95', display COUNT of rows WHERE hire_date LIKE '%96'

谁能指导我如何实现这一目标？

Answer 1

SELECT (SELECT COUNT(*) FROM employees WHERE hire_date = '1995') "1995",
       (SELECT COUNT(*) FROM employees WHERE hire_date = '1996') "1996"
FROM dual;

Answer 2

使用此查询，您只能访问员工一次 table

select  
  sum(case TO_CHAR(hire_date, 'YYYY') when '1995' then 1 end) as "1995",
  sum(case TO_CHAR(hire_date, 'YYYY') when '1996' then 1 end) as "1996"
  from employees
 where hire_date between date '1995-01-01' and date'1996-12-31';

SQL - 根据 2 个限制计算行数

SQL - Count rows based on 2 restrictions

oracle

oracle10g