SELECT * FROM Dogs įurthermore, I will continue to get this error, even if I delete Maximus from the table (i.e. I got this error because the largest possible ROWID is already being used in the table, and because this table was created with the AUTOINCREMENT keyword, it will not go back and search for an unused ROWID value. So we get a different result to the Cats table. Now let’s try to insert a new row to the Dogs table: INSERT INTO Dogs VALUES ( NULL, 'Lickable' ) Without having any other column to record the date/time that the row was inserted/updated, one might incorrectly assume that Magnus was inserted after Scratchy. However, notice that the new auto-incrementing value is lower than the previous value (it has no other option). Insert into the Cats table: INSERT INTO Cats VALUES ( NULL, 'Scratchy' ) Let’s see what happens when I try to insert a new row into each table (without explicitly specifying a value for the auto-incrementing columns). OK, so both tables use the largest possible integer as their largest ROWID values. We can take the previous example a step further and insert a new row by explicitly using the maximum ROWID possible. It simply increments to the next value, leaving a gap in the numbering. It was created with the AUTOINCREMENT keyword and therefore it can’t reuse the previous value. To recap, the Cats table was created without the AUTOINCREMENT keyword, and the Dogs table was created with the AUTOINCREMENT keyword.Īfter deleting the last row from the Cats table, the next INSERT operation resulted in the same ROWID being reused for that row. INSERT INTO Dogs VALUES ( NULL, 'New Fluff' ) INSERT INTO Cats VALUES ( NULL, 'New Flutter' ) Now let’s delete the last row in each table, insert the rows again, then select the result: DELETE FROM Cats WHERE CatId = 3 Here’s an example to demonstrate the difference between implicitly and explicitly defining an auto-increment column. It just means that they will never decrease. This doesn’t meant that the values will always increment by 1 though. In other words, you can rely on this method for having ROWIDs in ascending order. Therefore using this method guarantees that the ROWIDs are monotonically increasing. Any attempt to insert a new row will fail with an SQLITE_FULL error. Once the largest possible ROWID has been inserted, no new inserts are allowed. In other words, it won’t go back and reuse previously deleted ROWID values. When you use the AUTOINCREMENT keyword, the ROWID chosen for the new row is at least one larger than the largest ROWID that has ever before existed in that same table. When you use this method, a slightly different algorithm is used to determine the auto-incremented value. To do this, use the AUTOINCREMENT keyword. You can also create auto-incrementing columns explicitly. However, as long as you never use the maximum ROWID value and you never delete the entry in the table with the largest ROWID, this method will generate monotonically increasing unique ROWIDs. When you omit the AUTOINCREMENT keyword, once ROWID is equal to the largest possible integer (9223372036854775807), SQLite will try to find an unused positive ROWID at random. This is due to the way auto-incrementing works when omitting the AUTOINCREMENT keyword vs using that keyword. One benefit of omitting the AUTOINCREMENT keyword is that it reduces CPU, memory, disk space, and disk I/O overhead.Īn important downside however, is that you can’t guarantee that all rows will be incremented in ascending order. You can access the ROWID value by using any of four names the column name, ROWID, _ROWID_, or OID. In other words, if you insert NULL into that column, it will be converted to the current ROWID.Īctually, the way it works is that the column becomes an alias for the ROWID. When you do this, any NULL values are converted into the the current ROWID. Therefore, you don’t actually need to use the AUTOINCREMENT keyword to have a column that uses an automatically incrementing value for each row. When you declare a column as INTEGER PRIMARY KEY, it will automatically increment. However, there are some subtle differences between how each method works. One downside of this method is that it uses extra CPU, memory, disk space, and disk I/O overhead.īoth methods cause the column to use an incrementing value each time a new row is inserted with NULL in that column. You can create it explicitly with the AUTOINCREMENT keyword.You can create it implicitly when you define the column as INTEGER PRIMARY KEY.There are a couple of ways you can create an AUTOINCREMENT column: In SQLite, an AUTOINCREMENT column is one that uses an automatically incremented value for each row that’s inserted into the table.
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