01.Primary Key
Primary key can be defined as a set of one or more fields/columns of a table that uniquely identify a record in database table.
02.select * from database. table；
03.group by
-summarize data by selected fields
false case：
correct：
select zip， max（price）
from Sacramento
group by zip；
04.errors in progarmming
05.*all fields without transformation should be added in group by（）；
06.having （）：filter on grouped data
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