Interview : update table values using select statement

Question

Interviewer asked me one question, which seems very easy, but I couldn't figure out, how to solve this

Name  | Gender
--------------
A     | F
B     | M
C     | F
D     | F
E     | M

From the above data, gender was wrongly entered, which means in place of F it should be M and in place of M it should F. How to update whole table with a single line sql query (don't use pl/sql block). Since, if I will update gender column one by one, then possible error would be all rows values of gender column becomes either F or M.

Final output should be

Name  | Gender
--------------
A     | M
B     | F
C     | M
D     | M
E     | F

Answer 1

Try this..

Update TableName Set Gender=Case when Gender='M' Then 'F' Else 'M' end

On OP request..update using Select...

Update TableName T Set Gender=(
Select Gender from TableName B where  T.Gender!=B.Gender and rownum=1);

SQL FIDDLE DEMO

Answer 2

update table_name
set    gender =  case when gender = 'F' then 'M' 
                      when gender = 'M' then 'F'
                  end

SQL works on Set theory principles, so updates are happening in parallel, you don't need Temporary storage to store the values before overwriting like we do in other programming language while swapping two values.