Following is an example of nested MySQL Case When or then else End statement.
select
CASE
When (table.field1 = '01')
THEN
CASE
When (SUBSTRING_INDEX(table.field1, ' ', 1) = 'abc')
THEN '01abc'
ELSE 'abc'
END
WHEN (table.field1 = '02')
THEN
CASE
When ((SUBSTRING_INDEX(table.field1, ' ', 1)) = 'def')
THEN '02def'
ELSE 'def'
END
When (table.field1 = '03')
THEN
CASE
When ((SUBSTRING_INDEX(table.field1, ' ', 1)) = 'efg')
THEN '03efg'
ELSE 'efg'
END
WHEN (table.field1 = '04')
THEN
CASE
When ((SUBSTRING_INDEX(table.field1, ' ', 1)) = 'pqr')
THEN '04pqr'
ELSE 'pqr'
END
END
from table ;
