在 SQL 中，如何使用 left 语句计算新创建的别名的不同值

Question

I probably didn't word the title properly, but basically I've created a column and I'm trying to count the distinct values within that column:

SELECT
    LEFT (route_id, 7) AS unique_route_plan,
    route_id,
    station_code,
    country_code,
    plan_wave
FROM
    perfectmile_na.d_perfectmile_routing
WHERE
    date BETWEEN '2020-01-01 00:00:00'
    AND '2020-12-31 00:00:00'
    AND station_code LIKE 'D%'
    AND plan_planning_algo <> 'DYNAMIC-PLANNER'

So in the above, I'd like to know how many distinct values of unique_route_plan there are.

Appreciate any help!

Answer 1

You can use the main query in a subquery like this and then do a count of distinct.

select count(distinct unique_route_plan) from
(SELECT
    LEFT (route_id, 7) AS unique_route_plan,
    route_id,
    station_code,
    country_code,
    plan_wave
FROM
    perfectmile_na.d_perfectmile_routing
WHERE
    date BETWEEN '2020-01-01 00:00:00'
    AND '2020-12-31 00:00:00'
    AND station_code LIKE 'D%'
    AND plan_planning_algo <> 'DYNAMIC-PLANNER')

Answer 2

You don't say how you want your data or provide an example of what you're looking for, however presumably you want a count of the distinct values as a column in the same query, so you can do

SELECT
    LEFT (route_id, 7) AS unique_route_plan,
    count(*) over (partition by left(route_id, 7)) as unique_route_plan_count,
    route_id,
    station_code,
    country_code,
    plan_wave
FROM
    perfectmile_na.d_perfectmile_routing
WHERE
    date BETWEEN '2020-01-01 00:00:00'
    AND '2020-12-31 00:00:00' -- << -- Are you sure??
    AND station_code LIKE 'D%'
    AND plan_planning_algo <> 'DYNAMIC-PLANNER'

And note your date range excludes the 31st December - that might be what you intended but I suspect not?

Answer 3

select count(distinct unique_route_plan) from
(
SELECT
    LEFT (route_id, 7) AS unique_route_plan,
    route_id,
    station_code,
    country_code,
    plan_wave
FROM
    perfectmile_na.d_perfectmile_routing
WHERE
    date BETWEEN '2020-01-01 00:00:00'
    AND '2020-12-31 00:00:00'
    AND station_code LIKE 'D%'
    AND plan_planning_algo <> 'DYNAMIC-PLANNER')t

Answer 4

I would simply use:

SELECT COUNT(DISTINCT LEFT(route_id, 7)) AS num_unique_route_plan
FROM perfectmile_na.d_perfectmile_routing
WHERE date BETWEEN '2020-01-01' AND '2020-12-31' AND
      station_code LIKE 'D%' AND
      plan_planning_algo <> 'DYNAMIC-PLANNER';

Note: If you want all of 2020, the logic would be:

WHERE date >= '2020-01-01' AND
      date < '2021-01-01' AND
      station_code LIKE 'D%' AND
      plan_planning_algo <> 'DYNAMIC-PLANNER';

I would advise you not to use BETWEEN on dates.