Postgres 10

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create table test (x int, y int);
insert into test values(1,2),(1,1),(2,1),(2,2),(null,null);

CREATE TABLE



INSERT 0 5


select 
  x,y,
  case 
    when x = 1 then
      case when y = 1 then 11
      else 12 
      end
    when x = 2 then
      case when y = 1 then 21
      else 22
      end
  else 99 end myExression
from test


x
y
myexression


1
2
12

1
1
11

2
1
21

2
2
22

null
null
99



SELECT 5


select 
  x,y,
  case
    when x = 1 and y = 1 then 11
    when x = 1 and y = 2 then 12
    when x = 2 and y = 1 then 21
    when x = 2 and y = 2 then 22
    else 99 
  end myExpression
from test


x
y
myexpression


1
2
12

1
1
11

2
1
21

2
2
22

null
null
99



SELECT 5

x	y	myexression
1	2	12
1	1	11
2	1	21
2	2	22
null	null	99

x	y	myexpression
1	2	12
1	1	11
2	1	21
2	2	22
null	null	99