MySQL计数不删除

Question

I have an sql query that simply adds a row to an existing database table. The first field is id and I assume this value has to be specified in the query.

So then id has to be the number of rows existing + 1. I'm determining the id like this:

SELECT COUNT(1) FROM testtable

The problem is that this returns the number of rows that have ever been added, including deleted ones. Because I have been adding and removing from this table, this number is greater than the number of EXISTING rows in the table which is what I want.

How can I count the existing rows in the table instead of the existing + deleted rows?

Answer 1

If possible switch to using an auto_increment column for your id and don't be concerned with gaps in the sequence of ids. Your own implementation of id generation may inflict more harm especially in a long run.

Now, back to your immediate question. You are probably looking for this

SELECT MAX(id) + 1 new_id 
  FROM Table1

Note: This query might fail under heavy load when several concurrent sessions issuing this query might grab the same id and your subsequent INSERT will fail. Therefore again consider using an auto_increment for your id.

Here is SQLFiddle demo

Answer 2

That's a bad idea, because you can end up with duplicate IDs, especially if you delete rows in the middle. If you're not going to use an auto-increment field, you can add the ID to the insert. Just use this in place of the value:

((SELECT MAX(t.id) + 1 FROM table t)

The full query would then be:

INSERT INTO table_name (id, col1, col2, col3) VALUES ((SELECT MAX(t.id) + 1 FROM table_name t), :col1, :col2, :col3)

Answer 3

SELECT COUNT(*) from table

将仅返回其中的条目数，而不基于ID

Answer 4

There is also possibility to find, and fill the gaps, by picking first free id

SELECT MIN(t1.id + 1) AS free_id
FROM t1
WHERE NOT EXISTS (SELECT t2.id FROM t2 WHERE t2.id = t1.id + 1)