Sqlite ALTER TABLE - add column between existing columns?

问题

If I have a table with columns: a, b, c and later I do a ALTER TABLE command to add a new column "d", is it possible to add it between a and b for example, and not at the end?

I heard that the position of the columns affects performance.

回答1:

It's not possible to add a column between two existing columns with an ALTER TABLE statement in SQLite. This works as designed.

The new column is always appended to the end of the list of existing columns.

As far as I know, MySQL is the only SQL (ish) dbms that lets you determine the placement of new columns.

To add a column at a specific position within a table row, use FIRST or AFTER col_name. The default is to add the column last. You can also use FIRST and AFTER in CHANGE or MODIFY operations to reorder columns within a table.

But this isn't a feature I'd use regularly, so "as far as I know" isn't really very far.

回答2:

With every sql platform I've seen the only way to do this is to drop the table and re-create it.

However, I question if the position of the column affects performance... In what way would it, what operations are you doing that you think it will make a difference?

I will also note that dropping the table and recreating it is often not a heavy lift. Making a backup of a table and restoring that table is easy on all major platforms so scripting a backup - drop - create - restore is an easy task for a competent DBA.

In fact I've done so often when users ask -- but I always find it a little silly. The most often reason given is the tool of choice behaves nicer when the columns are created in a certain order. (This was also @Jarad's reason below) So this is a good lesson for tool makers, make your tool able to reorder columns (and remember it between runs) -- then everyone is happy.

回答3:

I use the DB.compileStatement:

    sql = DB.compileStatement("INSERT INTO tableX VALUES (?,?,?);
    sql.bindString(1,"value for column 1");
    sql.bindString(2,"value for column 2");
    sql.bindString(3,"value for column 3");
    sql.executeUpdateDelete();

So there will be a big difference if order of the columns is not correct. Unfortunately adding columns at a specific position is not possible using ALTER TABLE, at least not in SQLite. (MySQL it is possible). Workaroud is recreating the table.. (and backup and restore data)

来源：https://stackoverflow.com/questions/24477765/sqlite-alter-table-add-column-between-existing-columns

标签

sql

performance

sqlite

alter-table