Small things
1.
<> Not equal to
select name,capital from world where left(name,1)=left(capital,1) and name<> capital
2.
select * from nobel where winner ='EUGENE O''NEILL'
#when you want to select someone with the name EUGENE O'NEILL
3.
count(*),min,max will IGNORE values
4.
a is null ##### not a=NULL
5.
NOT IN (2,2,2,NULL) is an empty set
6. Distinct (1,2,2,NULL) = 1,2,NULL
min(1,1,NULL) = 1
7. Dont NAME subquery Table inside an in statement
in( select countries from world) will work