查询 - 计算具有相同列的 2 个数据库表之间的差异

Question

How do you calculate the difference between 2 DB tables with the same columns using query? They both have Columns as 'app', 'installs', 'usage'. Thanks in advance.

My notworking query

WITH apps AS (
SELECT app, installs_curr, usage_curr FROM (
    SELECT app, installs AS installs_curr, usage AS usage_curr
    FROM e5a.apps_2020_07
    )
SELECT app, installs_prev, usage_prev FROM (
   SELECT app, installs AS installs_prev, usage AS usage_prev
    FROM e5a.apps_2020_06
    )
)
SELECT app, installs_curr – installs_prev AS installs_delta, usage_curr – usage_prev AS usage_delta FROM apps
ORDER BY usage_delta DESC

**TABLE e5a.apps_2020_07**

app   installs usage 
snap  20       10

**TABLE e5a.apps_2020_06**

app   installs usage 
snap  12       8
  
**Result through calculation**

app   installs usage 
snap  8       2

Answer 1

It depends on how you want to treat values in one table but not the other. A good place to start is with join :

SELECT a7.app,
       (a7.installs - a6.installs) AS installs_diff,
       (a7.usage - a6.usage) AS usage_diff
FROM e5a.apps_2020_07 a7 JOIN
     e5a.apps_2020_08 a6
     ON a7.apps = a6.apps;

This include only apps that are in both tables.

You should fix your data model so you have only one table with the month as a column in the table.

Answer 2

Assuming that App is primary key, then you can simply subtract:

SELECT a.app, a.installs - b.installs installs_curr, a.usage - b.usage  AS usage_curr
    FROM e5a.apps_2020_07 a inner join e5a.apps_2020_06 b on (a.app = b.app)