04-22-2008
So far so good. You've connected to the DB...
Is the deletion business logic simple enough to be done in a single DML statement? For example...
DELETE from TABLENAME T where T.SOMEDATE <= (sysdate() - 7) and T.FREEFLAG = TRUE;
I don't know mysql so I'm giving a generic kind of syntax here... If the issue is not that simple, then you need to issue an appropriate SELECT statement, capture the results in perl, analyze them, and then issue delete statements on the appropriate rows... I haven't used the DBI in a while, so I don't remember the exact syntax of passing DML statements to the DBI, but in PERL, the results of the statement can be captured automatically in an array... Again I'm going to illustrate with "pseudo syntax". You translate into something that actually works...
$statement_to_exec = "SELECT X,Y,Z from T where Z...;"
(@myARRAY) = $myconnect->execute($statement_to_exec);
Now suppose you have properly set up your select statement so that the data comes out like this....
A,B,C
D,E,F
G,H,I
etc... That is, fields separated by commas (just as an example) and each record on its own line... So guess what.... $myARRAY[0] = "A,B,C" and $myARRAY[1] = "D,E,F" etc. That is each record is put in a separate element of @myARRAY! I'm assuming here that you've left perl's default line ending character alone, that the lines end with that (usual) character, etc. You can control all of this, but usually it isn't necessary....
OK, so now you can iterate over each line...
while ($line = shift @myARRAY) { ... logic here ... }
And then, you can split line into it's separate fields with....
($field1, $field2, $field3) = split /,/ , $line;
Now $field1 = "A", $field2 = "B", etc.
This is how you proceede... When you decide which lines must be deleted, say the line in $myARRAY[2] (the third line), then you issue a DELETE via the DBI for that line only....
As for the cron line, that is very easy. Just edit a file in your home directory called myCRONTAB (sysadmins might want you to use a system CRONTAB, or some other established one) and put a line in it like....
0 0 * * * /path/to/your/script.pl (says run at 0 min of 0 hr every day, I'll let you look up cron syntax for yourself)...
Then set up the job by entering cron myCRONTAB and if there are no syntax errors in the file, your good to go...
Oh... One more thing to remember.... when you run a script from cron it has NO ENVIRONMENT... So, you can't rely on PATH, or anything else. You have to set all that stuff up in your perl script....
Long I know, but I hope it helps....
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DELETE(7) SQL Commands DELETE(7)
NAME
DELETE - delete rows of a table
SYNOPSIS
DELETE FROM [ ONLY ] table [ [ AS ] alias ]
[ USING usinglist ]
[ WHERE condition | WHERE CURRENT OF cursor_name ]
[ RETURNING * | output_expression [ [ AS ] output_name ] [, ...] ]
DESCRIPTION
DELETE deletes rows that satisfy the WHERE clause from the specified table. If the WHERE clause is absent, the effect is to delete all rows
in the table. The result is a valid, but empty table.
Tip: TRUNCATE [truncate(7)] is a PostgreSQL extension that provides a faster mechanism to remove all rows from a table.
By default, DELETE will delete rows in the specified table and all its child tables. If you wish to delete only from the specific table
mentioned, you must use the ONLY clause.
There are two ways to delete rows in a table using information contained in other tables in the database: using sub-selects, or specifying
additional tables in the USING clause. Which technique is more appropriate depends on the specific circumstances.
The optional RETURNING clause causes DELETE to compute and return value(s) based on each row actually deleted. Any expression using the
table's columns, and/or columns of other tables mentioned in USING, can be computed. The syntax of the RETURNING list is identical to that
of the output list of SELECT.
You must have the DELETE privilege on the table to delete from it, as well as the SELECT privilege for any table in the USING clause or
whose values are read in the condition.
PARAMETERS
ONLY If specified, delete rows from the named table only. When not specified, any tables inheriting from the named table are also pro-
cessed.
table The name (optionally schema-qualified) of an existing table.
alias A substitute name for the target table. When an alias is provided, it completely hides the actual name of the table. For example,
given DELETE FROM foo AS f, the remainder of the DELETE statement must refer to this table as f not foo.
usinglist
A list of table expressions, allowing columns from other tables to appear in the WHERE condition. This is similar to the list of
tables that can be specified in the FROM Clause [select(7)] of a SELECT statement; for example, an alias for the table name can be
specified. Do not repeat the target table in the usinglist, unless you wish to set up a self-join.
condition
An expression that returns a value of type boolean. Only rows for which this expression returns true will be deleted.
cursor_name
The name of the cursor to use in a WHERE CURRENT OF condition. The row to be deleted is the one most recently fetched from this cur-
sor. The cursor must be a non-grouping query on the DELETE's target table. Note that WHERE CURRENT OF cannot be specified together
with a Boolean condition. See DECLARE [declare(7)] for more information about using cursors with WHERE CURRENT OF.
output_expression
An expression to be computed and returned by the DELETE command after each row is deleted. The expression can use any column names
of the table or table(s) listed in USING. Write * to return all columns.
output_name
A name to use for a returned column.
OUTPUTS
On successful completion, a DELETE command returns a command tag of the form
DELETE count
The count is the number of rows deleted. If count is 0, no rows matched the condition (this is not considered an error).
If the DELETE command contains a RETURNING clause, the result will be similar to that of a SELECT statement containing the columns and val-
ues defined in the RETURNING list, computed over the row(s) deleted by the command.
NOTES
PostgreSQL lets you reference columns of other tables in the WHERE condition by specifying the other tables in the USING clause. For exam-
ple, to delete all films produced by a given producer, one can do:
DELETE FROM films USING producers
WHERE producer_id = producers.id AND producers.name = 'foo';
What is essentially happening here is a join between films and producers, with all successfully joined films rows being marked for dele-
tion. This syntax is not standard. A more standard way to do it is:
DELETE FROM films
WHERE producer_id IN (SELECT id FROM producers WHERE name = 'foo');
In some cases the join style is easier to write or faster to execute than the sub-select style.
EXAMPLES
Delete all films but musicals:
DELETE FROM films WHERE kind <> 'Musical';
Clear the table films:
DELETE FROM films;
Delete completed tasks, returning full details of the deleted rows:
DELETE FROM tasks WHERE status = 'DONE' RETURNING *;
Delete the row of tasks on which the cursor c_tasks is currently positioned:
DELETE FROM tasks WHERE CURRENT OF c_tasks;
COMPATIBILITY
This command conforms to the SQL standard, except that the USING and RETURNING clauses are PostgreSQL extensions.
SQL - Language Statements 2010-05-14 DELETE(7)