แผนภูมิวงกลม แต่ละกองกำลังตำรวจมีกี่มณฑล?ไม่จำเป็นต้องใช้ตารางทั้งหมด
Pie chart. how many counties correspond to each police force?It is not neccessary to use all the tables.
 city:  City_ID [ int]  County_ID [ int]  Name [ text]  White [ real]  Black [ real]  Amerindian [ real]  Asian [ real]  Multiracial [ real]  Hispanic [ real] 
SELECT Police_force, COUNT(*) FROM county_public_safety GROUP BY Police_force