表数据

备注：

• id: 任务id;
• name: 参与人name;

1：distinct

1.1

-- 根据任务ID去重
SELECT DISTINCT id FROM test;

1.2

-- 任务总数
SELECT COUNT(DISTINCT id) FROM test;

1.3

distinct 通常效率较低。它不适合用来展示去重后具体的值，一般与 count 配合用来计算条数。
distinct 使用中，放在 select 后边，对后面所有的字段的值统一进行去重。比如distinct后面有两个字段，那么 11,11 和 11, 21 这两条记录不是重复值

SELECT DISTINCT id, name FROM test;

2: group by

2.1

SELECT id, name, count(*) FROM test
GROUP BY id;

-- 任务总数
SELECT
	count( tmp.id ) 
FROM
	( SELECT id, NAME FROM test GROUP BY id ) tmp

3：row_number

row_number 是窗口函数，语法如下：
row_number() over (partition by <用于分组的字段名> order by <用于组内排序的字段名>) 其中partition by 部分可省略

SELECT 
	id,
	name,
	ROW_NUMBER() over (ORDER BY id) rn
FROM test

SELECT 
	id,
	name,
	ROW_NUMBER() over (PARTITION by id ORDER BY id) rn
FROM test

SELECT
	COUNT( CASE WHEN rn = 1 THEN id ELSE NULL END ) count 
FROM
	( SELECT id, NAME, ROW_NUMBER() over ( PARTITION BY id ORDER BY id ) rn FROM test ) tmp