Firebird 2.1 Language Reference Update

Syntax:
This is for experts only!
DML statements
The need for a pessimistic lock in Firebird is very rare indeed and should be well understood before use of
this extension is considered.
It is essential to understand the effects of transaction isolation and other transaction attributes before attempting
to implement explicit locking in your application.
SELECT ... FROM single_table
[WHERE ...]
[FOR UPDATE [OF ...]]
WITH LOCK
If the WITH LOCK clause succeeds, it will secure a lock on the selected rows and prevent any other transaction
from obtaining write access to any of those rows, or their dependants, until your transaction ends.
If the FOR UPDATE clause is included, the lock will be applied to each row, one by one, as it is fetched into
the server-side row cache. It becomes possible, then, that a lock which appeared to succeed when requested
will nevertheless fail subsequently, when an attempt is made to fetch a row which becomes locked by another
transaction.
WITH LOCK can only be used with a top-level, single-table SELECT statement. It is not available:
• in a subquery specification;
• for joined sets;
• with the DISTINCT operator, a GROUP BY clause or any other aggregating operation;
• with a view;
• with the output of a selectable stored procedure;
• with an external table.
A lengthier, more in-depth discussion of “SELECT ... WITH LOCK” is included in the Notes. It is a must-read
for everybody who considers using this feature.
Available in: DSQL, ESQL, PSQL
UPDATE
Description: Changes values in a table (or in one or more tables underlying a view). The columns affected are
specified in the SET clause; the rows affected may be limited by the WHERE and ROWS clauses.
Syntax:
UPDATE [TRANSACTION name] {tablename | viewname} [[AS] alias]
SET col = newval [, col = newval ...]
[WHERE {search-conditions | CURRENT OF cursorname}]
[PLAN plan_items]
[ORDER BY sort_items]
[ROWS [TO ]]
[RETURNING values [INTO ]]
, ::= Any expression evaluating to an integer.
::= :varname [, :varname ...]
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