我想在sqlite数据库中删除或添加列
我正在使用以下查询删除列。
ALTER TABLE TABLENAME DROP COLUMN COLUMNNAME
但它会产生错误
System.Data.SQLite.SQLiteException: SQLite error
near "DROP": syntax error
我想在sqlite数据库中删除或添加列
我正在使用以下查询删除列。
ALTER TABLE TABLENAME DROP COLUMN COLUMNNAME
但它会产生错误
System.Data.SQLite.SQLiteException: SQLite error
near "DROP": syntax error
当前回答
这个不同问题的答案是针对修改列的,但我相信,如果你有很多列,并且不想在INSERT语句中手动重新输入大部分列,部分答案也可以产生有用的方法:
https://stackoverflow.com/a/10385666
您可以按照上面链接中描述的方法转储数据库,然后从该转储中获取“create table”语句和“insert”模板,然后按照SQLite FAQ条目“如何从SQLite中现有的表中添加或删除列”中的说明进行操作。(常见问题在本页其他地方有链接。)
其他回答
基于http://www.sqlite.org/faq.html#q11上的信息在Python中实现。
import sqlite3 as db
import random
import string
QUERY_TEMPLATE_GET_COLUMNS = "PRAGMA table_info(@table_name)"
QUERY_TEMPLATE_DROP_COLUMN = """
BEGIN TRANSACTION;
CREATE TEMPORARY TABLE @tmp_table(@columns_to_keep);
INSERT INTO @tmp_table SELECT @columns_to_keep FROM @table_name;
DROP TABLE @table_name;
CREATE TABLE @table_name(@columns_to_keep);
INSERT INTO @table_name SELECT @columns_to_keep FROM @tmp_table;
DROP TABLE @tmp_table;
COMMIT;
"""
def drop_column(db_file, table_name, column_name):
con = db.connect(db_file)
QUERY_GET_COLUMNS = QUERY_TEMPLATE_GET_COLUMNS.replace("@table_name", table_name)
query_res = con.execute(QUERY_GET_COLUMNS).fetchall()
columns_list_to_keep = [i[1] for i in query_res if i[1] != column_name]
columns_to_keep = ",".join(columns_list_to_keep)
tmp_table = "tmp_%s" % "".join(random.sample(string.ascii_lowercase, 10))
QUERY_DROP_COLUMN = QUERY_TEMPLATE_DROP_COLUMN.replace("@table_name", table_name)\
.replace("@tmp_table", tmp_table).replace("@columns_to_keep", columns_to_keep)
con.executescript(QUERY_DROP_COLUMN)
con.close()
drop_column(DB_FILE, TABLE_NAME, COLUMN_NAME)
这个脚本首先创建随机的临时表,并只插入必要列的数据,除了将要删除的列。然后根据临时表恢复原表,删除临时表。
作为替代:
如果有一个带模式的表
CREATE TABLE person(
id INTEGER PRIMARY KEY,
first_name TEXT,
last_name TEXT,
age INTEGER,
height INTEGER
);
你可以使用CREATE TABLE…AS语句,如CREATE TABLE person2 AS SELECT id, first_name, last_name, age FROM person;,即去掉你不想要的列。然后删除原来的person表并重命名新表。
注意,此方法生成的表没有PRIMARY KEY,也没有约束。为了保留这些表,可以使用其他人描述的方法来创建新表,或者使用临时表作为中间表。
SQLite 3.35.0引入了对ALTER TABLE DROP COLUMN的支持。
ALTER TABLE DROP COLUMN语法用于从表中删除现有列。DROP COLUMN命令从表中删除指定的列,并重写整个表以清除与该列相关的数据。DROP COLUMN命令仅在列未被模式的任何其他部分引用、不是PRIMARY KEY且没有UNIQUE约束时才有效。
下面的语法是有效的:
ALTER TABLE <TABLENAME> DROP COLUMN <COLUMNNAME>;
ALTER TABLE <TABLENAME> DROP <COLUMNNAME>;
我重写了@Udinic答案,以便代码自动生成表创建查询。它也不需要ConnectionSource。它还必须在事务中完成此操作。
public static String getOneTableDbSchema(SQLiteDatabase db, String tableName) {
Cursor c = db.rawQuery(
"SELECT * FROM `sqlite_master` WHERE `type` = 'table' AND `name` = '" + tableName + "'", null);
String result = null;
if (c.moveToFirst()) {
result = c.getString(c.getColumnIndex("sql"));
}
c.close();
return result;
}
public List<String> getTableColumns(SQLiteDatabase db, String tableName) {
ArrayList<String> columns = new ArrayList<>();
String cmd = "pragma table_info(" + tableName + ");";
Cursor cur = db.rawQuery(cmd, null);
while (cur.moveToNext()) {
columns.add(cur.getString(cur.getColumnIndex("name")));
}
cur.close();
return columns;
}
private void dropColumn(SQLiteDatabase db, String tableName, String[] columnsToRemove) {
db.beginTransaction();
try {
List<String> columnNamesWithoutRemovedOnes = getTableColumns(db, tableName);
// Remove the columns we don't want anymore from the table's list of columns
columnNamesWithoutRemovedOnes.removeAll(Arrays.asList(columnsToRemove));
String newColumnNamesSeparated = TextUtils.join(" , ", columnNamesWithoutRemovedOnes);
String sql = getOneTableDbSchema(db, tableName);
// Extract the SQL query that contains only columns
String oldColumnsSql = sql.substring(sql.indexOf("(")+1, sql.lastIndexOf(")"));
db.execSQL("ALTER TABLE " + tableName + " RENAME TO " + tableName + "_old;");
db.execSQL("CREATE TABLE `" + tableName + "` (" + getSqlWithoutRemovedColumns(oldColumnsSql, columnsToRemove)+ ");");
db.execSQL("INSERT INTO " + tableName + "(" + newColumnNamesSeparated + ") SELECT " + newColumnNamesSeparated + " FROM " + tableName + "_old;");
db.execSQL("DROP TABLE " + tableName + "_old;");
db.setTransactionSuccessful();
} catch {
//Error in between database transaction
} finally {
db.endTransaction();
}
}
我根据Sqlite推荐的方法写了一个Java实现:
private void dropColumn(SQLiteDatabase db,
ConnectionSource connectionSource,
String createTableCmd,
String tableName,
String[] colsToRemove) throws java.sql.SQLException {
List<String> updatedTableColumns = getTableColumns(tableName);
// Remove the columns we don't want anymore from the table's list of columns
updatedTableColumns.removeAll(Arrays.asList(colsToRemove));
String columnsSeperated = TextUtils.join(",", updatedTableColumns);
db.execSQL("ALTER TABLE " + tableName + " RENAME TO " + tableName + "_old;");
// Creating the table on its new format (no redundant columns)
db.execSQL(createTableCmd);
// Populating the table with the data
db.execSQL("INSERT INTO " + tableName + "(" + columnsSeperated + ") SELECT "
+ columnsSeperated + " FROM " + tableName + "_old;");
db.execSQL("DROP TABLE " + tableName + "_old;");
}
为了获得表的列,我使用了“PRAGMA table_info”:
public List<String> getTableColumns(String tableName) {
ArrayList<String> columns = new ArrayList<String>();
String cmd = "pragma table_info(" + tableName + ");";
Cursor cur = getDB().rawQuery(cmd, null);
while (cur.moveToNext()) {
columns.add(cur.getString(cur.getColumnIndex("name")));
}
cur.close();
return columns;
}
实际上我在我的博客上写过,你可以在那里看到更多的解释:
http://udinic.wordpress.com/2012/05/09/sqlite-drop-column-support/