如何编写与计数相关的查询

Question

I have two tables such as Car and Driver . Car table has two columns. which are car_id and car_name . Driver table also has two columns. which are driver_id and car_type .

Cars

car_id	car_name
1	Audi
2	BMW
3	Ferrari

Drivers

driver_id	car_type
1	BMW
1	Audi
1	Ferrari
2	Audi
2	Cheverolet
2	Tata
3	Ferrari

How do I find which driver is driving more than two cars?

Answer 1

By using the Having clause

SELECT Driver_id
FROM Driver
GROUP BY Driver_id
HAVING COUNT(*) > 1

Answer 2

Your tables have surrogate primary keys but the Driver table uses the car name, not id, to relate cars to drivers.

So you can find the drivers with more than one car without needing to join to the car table (unless you need the car_id value)

The query needs to count the distinct type of car grouped by driver_id , having a count of car type greater than one.

From the information in that sentence you should be able to look up how to write it in SQL ;)

If the Driver table has a unique constraint on the two fields driver_id / car_type then you can use the query Dijkgraaf posted and not worry about the type of car they drive.

From the data you have shown there can't be any referential constraint (foreign key) of the cars drivers drive because you have Cheverolet (sic), Tata , and Ferrari in the Driver table but those cars are not in the Car table.

Answer 3

SELECT Driver_id
FROM YourTable
Where COUNT(Driver_id) > 1