SELECT last id, without INSERT

I'm t开发者_运维技巧rying to retrieve the id of one table A to insert into another table B. I cannot use last_insert_id() as i have not inserted anything into A. Any ideas on how to do this nicely?

$n = mysql_query("SELECT max(id) FROM tablename"); doesn't seem to work, nor does

$n = mysql_query("SELECT max(id) FROM tablename GROUP BY id");

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In MySQL, this does return the highest value from the id column:

SELECT MAX(id) FROM tablename;

However, this does not put that id into $n:

$n = mysql_query("SELECT max(id) FROM tablename");

To get the value, you need to do this:

$result = mysql_query("SELECT max(id) FROM tablename");

if (!$result) {
    die('Could not query:' . mysql_error());
}

$id = mysql_result($result, 0, 'id');

If you want to get the last insert ID from A, and insert it into B, you can do it with one command:

INSERT INTO B (col) SELECT MAX(id) FROM A;

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You could descendingly order the tabele by id and limit the number of results to one:

SELECT id FROM tablename ORDER BY id DESC LIMIT 1

BUT: ORDER BY rearranges the entire table for this request. So if you have a lot of data and you need to repeat this operation several times, I would not recommend this solution.

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I have different solution:

SELECT AUTO_INCREMENT - 1 as CurrentId FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_SCHEMA = 'dbname' AND TABLE_NAME = 'tablename'

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You can get maximum column value and increment it:

InnoDB uses the following algorithm to initialize the auto-increment counter for a table t that contains an AUTO_INCREMENT column named ai_col: After a server startup, for the first insert into a table t, InnoDB executes the equivalent of this statement:

SELECT MAX(ai_col) FROM t FOR UPDATE;

InnoDB increments by one the value retrieved by the statement and assigns it to the column and to the auto-increment counter for the table. If the table is empty, InnoDB uses the value 1.

Also you can use SHOW TABLE STATUS and its "Auto_increment" value.

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I think to add timestamp to every record and get the latest. In this situation you can get any ids, pack rows and other ops.