Do the same operation as ALTER TABLE table DROP columns in SQLite3

sqlite3_drop_columns.tcl

package require sqlite3

proc format_sql {sql} {
	set sql [regsub -all -line -- {--.+$} $sql ""]
	set sql [string map [list "\n" ""] $sql]
	set sql [string map [list "," ",\n"] $sql]
	set sql [string map [list "(" "(\n"] $sql]
	set sql [string map [list ")" "\n)"] $sql]
	return $sql
}

proc get_column_names {db table} {
	set pragma [format {PRAGMA table_info('%s')} $table]
	set names [list]
	foreach {cid name type notnull dflt_value pk} [$db eval $pragma] {
		lappend names $name
	}
	return $names
}

proc drop_columns {db table drop_columns} {
	set orig_columns [get_column_names $db $table]
	set keep_columns [list]
	foreach column $orig_columns {
		if {$column ni $drop_columns} {
			lappend keep_columns $column
		}
	}
	set keep_columns_csv [join $keep_columns ,]
	
	set sql [$db onecolumn {SELECT sql FROM sqlite_master WHERE name = $table}]
	set sql [format_sql $sql]
	
	set keep_lines [split $sql \n]
	foreach drop_column $drop_columns {
		set keep_lines [lsearch -all -inline -not $keep_lines "*${drop_column}*"]
	}
	set create_sql [join $keep_lines \n]
	
	set unq [clock seconds]
	
	set old "${table}"
	set tmp "${table}_temp_$unq"
	
	$db eval "ALTER TABLE $old RENAME TO $tmp"
	$db eval $create_sql
	
	$db eval "INSERT INTO $old ($keep_columns_csv) SELECT $keep_columns_csv FROM $tmp"
	$db eval "DROP TABLE $tmp"
}

proc cleanup {db} {
	$db eval {VACUUM}
	$db eval {REINDEX}
}

if {[info exists ::argv0] && $::argv0 eq [info script]} {
	set dbfile "app.db"
	sqlite3 db $dbfile
	db transaction {
		drop_columns db "users" "temp"
	}
	cleanup db
	db close
	exit
}