MySQL Lock tables and subqueries

Example error:

Table 'table' was not locked with LOCK TABLES

Example:

LOCK TABLES x write, y write;

select * from x; #works

select * from y; #works

But:

select *,

(select fieldname from y where fieldcond=x.field2) as yname

from x where field1='xxx'; #failed with

#y tables is not locked

Solution:

LOCK TABLES x write, y write, y as yalias write;

select *,

(select fieldname from y yalias where fieldcond=x.field2) as yname

from x where field1='xxx';

#WORKS!