计算字段

SELECT
  Name,
  Population, 
  CASE WHEN Population > 1000000 THEN
     'Big city' ELSE
     'Small city' END CitySize
FROM city


Name Population CitySize
Kabul 1,780,000 Big city
Qandahar 237,500 Small city
Herat 186,800 Small city
Mazar-e-Sharif 127,800 Small city
Amsterdam 731,200 Small city
Rotterdam 593,321 Small city
Haag 440,900 Small city
Utrecht 234,323 Small city
Eindhoven 201,843 Small city
...

CASE WHEN 之后写条件：条件成立时，列取 THEN 后面的值；条件不成立时，列取 ELSE 后面的值。