Re: [SQL] How match percent sign in SELECT using LIKE?

Dan Lauterbach <danla(at)micromotion(dot)com> wrote:

> How do I match '%' in a SELECT query using LIKE predicate? For
> example, to query for DocNo's containing string 'EW%':
>
> SELECT * FROM XXXX WHERE DocNo LIKE '%EW%%';
>
> PostgreSQL wants to treat the '%' in 'EW%' as a wildcard. I've tried
> escaping the '%' using '\%',
> '%%'. The SQL-92 standard provides for this using the ESCAPE keyword:
>
> SELECT * FROM XXXX WHERE DocNo LIKE '%EW#%%' ESCAPE '#';

You apparently *can* use the '%' itself to mask the '%'. I read that
somewhere, but I don't find it in PostgreSQL's documentation now.

db=> create table xxx (x text);
db=> insert into xxx (x) values ('aaabbbccc');
db=> insert into xxx (x) values ('aaabbb%ccc');
db=> insert into xxx (x) values ('aaabbb%%ccc');

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Normal use of % as wildcard:

db=> select * from xxx where x like 'aaa%' order by 1;
x
-----------
aaabbb%%ccc
aaabbb%ccc
aaabbbccc
(3 rows)
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Escaping ...

db=> select * from xxx where x like 'aaabbb%%ccc' order by 1;
x
----------
aaabbb%ccc
(1 row)

db=> select * from xxx where x like 'aaabbb%%%%ccc' order by 1;
x
-----------
aaabbb%%ccc
(1 row)

db=> select * from xxx where x like 'aaabbb%%%%%%ccc' order by 1;
x
-
(0 rows)
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
This here I consider strange (shouldn't it only escape and not
'wildcard' additionally?!)

db=> select * from xxx where x like 'aaabbb%%' order by 1;
x
-----------
aaabbb%%ccc
aaabbb%ccc
(2 rows)

db=> select * from xxx where x like 'aaabbb%%c' order by 1;
x
----------
aaabbb%ccc
(1 row)

db=> select * from xxx where x like 'aaabbb%%cc' order by 1;
x
----------
aaabbb%ccc
(1 row)

db=> select * from xxx where x like 'aaabbb%%ccc' order by 1;
x
----------
aaabbb%ccc
(1 row)

db=> select * from xxx where x like 'aaabbb%%cccc' order by 1;
x
-
(0 rows)
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Anyway, you can use in PostgreSQL regular expressions as well:

=> select * from xxx where x ~ 'aa*b{3}%c+' order by 1;
x
----------
aaabbb%ccc
(1 row)

Much better, if you know regexps. But if I remember correctly, only
'LIKE ...'- and regular expressions which begin with a constant,
not-wildcard-part can be used for indexed search (other people
certainly know that much better than me ;-)

Tsch, Ulf

--
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Ulf Mehlig <umehlig(at)zmt(dot)uni-bremen(dot)de>
Center for Tropical Marine Ecology/ZMT, Bremen, Germany
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