Question

在某些数据库的表中使用一些SQL，我得到这样的结果：

id  name    date            status
1   John    2018-05-03      PRESENT
2   Mary    2018-05-03      NOT PRESENT
3   Jane    2018-05-03      NOT PRESENT
2   Mary    2018-05-04      PRESENT
1   John    2018-05-04      PRESENT
1   John    2018-05-05      PRESENT
2   Mary    2018-05-05      NOT PRESENT
3   Jane    2018-05-04      PRESENT
3   Jane    2018-05-05      NOT PRESENT
1   John    2018-05-06      PRESENT

我想进一步使用SQL来获得类似这样的结果：

id  name    date      present   not present
1   John    2018-05       4           0
2   Mary    2018-05       1           2
3   Jane    2018-05       2           1

换句话说，我想根据他/她每天收到的状态来提取学生在给定月份中参加了多少课。我该如何实现？

我正在使用Postgresql数据库。

Answer 1

使用conditional aggregation：

select id, name, to_char(date,'YYYY-MM') as "Date", 
       sum(case when status = 'PRESENT' then 1 else 0 end ) as present,
       sum(case when status = 'NOT PRESENT' then 1 else 0 end ) as not_present
  from tab
 group by id, name, "Date"
 order by id

Demo

保持else 0对于null返回案例中获得0很重要
列别名可能在group by列表中使用用于Postgres
由于需要的输出，将日期值截断为月需要to_char(date,'YYYY-MM')

Answer 2

"build:server:prod": "webpack --config webpack.server.prod.config.js",
"build:client:prod": "webpack --config webpack.client.prod.config.js",
"start:server:prod": "export NODE_ENV=production && node ./dist/server.*.js",
"start:iso:prod": "npm run build:client:prod && npm run build:server:prod && npm run start:server:prod"

Answer 3

使用条件聚合（使用export const saveChosenCompany = (username, company) => dispatch => { AsyncStorage.multiSet([ [`${username}.companyID`, company.id], [`${username}.companyName`, company.name] ]) .then(() => { dispatch({ type: SAVED_COMPANY, payload: company }) }) .catch(err => { console.log(err); dispatch({ type: SAVED_COMPANY, payload: { id: null, name: null } }) }) } export const initCompany = (company) => dispatch => { dispatch({ type: INIT_COMPANY, payload: company }) }）和filter：

date_trunc()

计算并添加在某些日期出现的不同值

3 个答案: