Commented by: @sim1984

select 1 as f
from rdb$database
where true = true is true

Work is correct

select 1 as f
from rdb$database
where true is true is true

Work is correct

select 1 as f
from rdb$database
where true = true = true

Invalid token.
Dynamic SQL Error.
SQL error code = -104.
Token unknown - line 3, column 19.
=.

Commented by: @pavel-zotov

One more sample, fails on recent FB versions:

show version;

select 1 x4a
from rdb$database
where not false and not false is not false;


select 1 x4b
from rdb$database
where not false and not false = not not not false;


select 1 x4c
from rdb$database
where not false and not false is not not not false;

Output:

ISQL Version: WI-T3.0.0.31733 Firebird 3.0 Beta 2
Server version:
Firebird/Windows/Intel/i386 (access method), version "WI-T3.0.0.31733 Firebird 3.0 Beta 2"
Firebird/Windows/Intel/i386 (remote server), version "WI-T3.0.0.31733 Firebird 3.0 Beta 2/tcp (csprog)/P13"
Firebird/Windows/Intel/i386 (remote interface), version "WI-T3.0.0.31733 Firebird 3.0 Beta 2/tcp (csprog)/P13"
on disk structure version 12.0

     X4A

============
1

     X4B

============
1

Statement failed, SQLSTATE = 42000
Dynamic SQL Error
-SQL error code = -104
-Token unknown - line 3, column 42
-not
At line 13 in file c4177e.sql

Commented by: @pavel-zotov

Reopened - see issue dated 27/Mar/15 01:45 PM.

One more sample, now with BETWEEN operator:

recreate table test1(id int, x boolean);
insert into test1 values(107, false);
insert into test1 values(109, false);
insert into test1 values(117, true );
insert into test1 values(121, true );
insert into test1 values(122, false);
insert into test1 values(128, true );
insert into test1 values(137, false);
insert into test1 values(144, false);

Then:

select * from test1 where x between true and true;

      ID       X 

============ =======
117 <true>
121 <true>
128 <true>
// OK.

select * from test1 where x between true and not false;
Statement failed, SQLSTATE = 42000
Dynamic SQL Error
-SQL error code = -104
-Token unknown - line 1, column 46
-not
At line 15 in file c4227.sql

select * from test1 where x between not false and true;
Statement failed, SQLSTATE = 42000
Dynamic SQL Error
-SQL error code = -104
-Token unknown - line 1, column 37
-not
At line 17 in file c4227.sql

select * from test1 where x between (not false) and (not false);

      ID       X 

============ =======
117 <true>
121 <true>
128 <true>

But may be this is not bug and usage of parenthesis is mandatory for such cases ?

Commented by: @asfernandes

There is operator IS [NOT] {TRUE | FALSE | UNKNOWN}

There is no operator IS [NOT] <expression> for IS NOT NOT ...