IN versus EXIST

EXISTS is more faster than IN because EXISTS returns a Boolean value whereas IN returns a value.

DIfference between varchar and nvarchar in Oracle

varchar2(20 char) means you can store 20 characters -- whereas varchar2(20) means you can store 20 bytes. the varchar2(20 char) might take 20, 40, 60, 80 or more bytes to hold a string. The maximum length of a varchar2 is still 4000 bytes so varchar2(4000 char) is "misleading" in that the field will max out at 4000 bytes -- not 4000 characters.