how to optimize away common subexpressions? - sqlite

Question

select x+y as z,
case
when "x"+"y" < 0 then "Less Than Zero"
when "x"+"y" > 0 then "Non Zero"
else "Zero"
end
from sometable;
Returns expected result, but the addition is done with each row of data multiple times.
I am trying to optimize the query as follows but not working..
select x+y as z,
case
when "z" < 0 then "Less Than Zero"
when "z" > 0 then "Non Zero"
else "Zero"
end
from sometable;
Always returns "Less Than Zero".
What am I doing wrong on this query? How can I avoid adding A and B multiple times while the query is being executed?

Answer 1

Column aliases in the SELECT clause are not available in other expressions in the same SELECT clause. (What should happen with SELECT x AS y, y AS x ...?)
You can make such an alias available by moving it into a subquery:
SELECT z,
CASE WHEN z < 0 THEN 'Less Than Zero'
WHEN z > 0 THEN 'Non Zero'
ELSE 'Zero'
END
FROM (SELECT x + y AS z
FROM sometable);
However, this only saves typing; it does not actually optimize away the duplicate computation:
sqlite> explain select z, z from (select x+y as z from sometable);
addr opcode p1 p2 p3 p4 p5 comment
---- ------------- ---- ---- ---- ------------- -- -------------
0 Init 0 11 0 00 Start at 11
1 OpenRead 1 2 0 2 00 root=2 iDb=0; sometable
2 Rewind 1 9 0 00
3 Column 1 0 3 00 r[3]=sometable.x
4 Column 1 1 4 00 r[4]=sometable.y
5 Add 4 3 1 00 r[1]=r[4]+r[3]
6 Add 4 3 2 00 r[2]=r[4]+r[3]
7 ResultRow 1 2 0 00 output=r[1..2]
8 Next 1 3 0 01
9 Close 1 0 0 00
10 Halt 0 0 0 00
11 Transaction 0 0 1 0 01 usesStmtJournal=0
12 TableLock 0 2 0 sometable 00 iDb=0 root=2 write=0
13 Goto 0 1 0 00