从SQLite表中删除列

时间：2011-05-09 14:09:15

标签： sql sqlite ddl

我遇到了问题：我需要从SQLite数据库中删除一列。我写了这个查询

alter table table_name drop column column_name

但它不起作用。请帮帮我。

12 个答案:

答案 0 :(得分：176)

来自：http://www.sqlite.org/faq.html：

  

（11）如何在SQLite中添加或删除现有表中的列。

     

SQLite限制了可用于添加的ALTER TABLE支持   列到表的末尾或更改表的名称。如果你   想要在表的结构中进行更复杂的更改，你   将不得不重新创建表。您可以将现有数据保存到   临时表，删除旧表，创建新表，然后复制   数据从临时表中返回。

     

例如，假设您有一个名为“t1”的表，其中包含列名   “a”，“b”和“c”，并且您要从中删除“c”列   表。以下步骤说明了如何做到这一点：

BEGIN TRANSACTION;
CREATE TEMPORARY TABLE t1_backup(a,b);
INSERT INTO t1_backup SELECT a,b FROM t1;
DROP TABLE t1;
CREATE TABLE t1(a,b);
INSERT INTO t1 SELECT a,b FROM t1_backup;
DROP TABLE t1_backup;
COMMIT;

答案 1 :(得分：52)

不要删除备份表，只需将其重命名为... 

BEGIN TRANSACTION;
CREATE TABLE t1_backup(a,b);
INSERT INTO t1_backup SELECT a,b FROM t1;
DROP TABLE t1;
ALTER TABLE t1_backup RENAME TO t1;
COMMIT;

答案 2 :(得分：34)

为简单起见，为什么不从select语句创建备份表？

CREATE TABLE t1_backup AS SELECT a, b FROM t1;
DROP TABLE t1;
ALTER TABLE t1_backup RENAME TO t1;

答案 3 :(得分：7)

http://lists.osgeo.org/pipermail/grass-user/2006-January/031981.html

还有一个名为Sqliteman的工具，它提供了删除列的可视选项。

谢谢， Jignesh

答案 4 :(得分：3)

=＆gt;使用以下查询直接创建新表：

CREATE TABLE Table_name (Column_1 text,Column_2 text);

=＆gt;现在使用以下查询将数据从Existing_table插入table_name：

insert into Table_name (Column_1,Column_2) FROM Existing_Table;

=＆gt;现在通过以下查询删除Existing_table：

DROP Existing_Table;

答案 5 :(得分：3)

此选项仅在您可以在DB Browser for SQLite等数据库浏览器中打开数据库时有效。

在SQL浏览器中用于SQLite：

  1. 转到标签，＆＃34;数据库结构＆＃34;
  2. 选择表选择修改表（位于选项卡下）
  3. 选择要删除的列
  4. 单击“删除”字段，然后单击“确定”

答案 6 :(得分：3)

PRAGMA foreign_keys=off;

BEGIN TRANSACTION;

ALTER TABLE table1 RENAME TO _table1_old;

CREATE TABLE table1 (
( column1 datatype [ NULL | NOT NULL ],
  column2 datatype [ NULL | NOT NULL ],
  ...
);

INSERT INTO table1 (column1, column2, ... column_n)
  SELECT column1, column2, ... column_n
  FROM _table1_old;

COMMIT;

PRAGMA foreign_keys=on;

有关更多信息： https://www.techonthenet.com/sqlite/tables/alter_table.php

答案 7 :(得分：1)

对于SQLite3 c ++：

void GetTableColNames( tstring sTableName , std::vector<tstring> *pvsCols )
{
    UASSERT(pvsCols);

    CppSQLite3Table table1;

    tstring sDML = StringOps::std_sprintf(_T("SELECT * FROM %s") , sTableName.c_str() );



    table1 = getTable( StringOps::tstringToUTF8string(sDML).c_str() );

    for ( int nCol = 0 ; nCol < table1.numFields() ; nCol++ )
    {
        const char* pch1 = table1.fieldName(nCol);  

        pvsCols->push_back( StringOps::UTF8charTo_tstring(pch1));
    }
}


bool ColExists( tstring sColName )
{
    bool bColExists = true;

    try
    {
        tstring sQuery = StringOps::std_sprintf(_T("SELECT %s FROM MyOriginalTable LIMIT 1;") , sColName.c_str() );

        ShowVerbalMessages(false);

        CppSQLite3Query q = execQuery( StringOps::tstringTo_stdString(sQuery).c_str() );

        ShowVerbalMessages(true);
    }
    catch (CppSQLite3Exception& e)
    {
        bColExists = false;
    }

    return bColExists;
}

void DeleteColumns( std::vector<tstring> *pvsColsToDelete )
{
    UASSERT(pvsColsToDelete);

    execDML( StringOps::tstringTo_stdString(_T("begin transaction;")).c_str() );


    std::vector<tstring> vsCols;
    GetTableColNames( _T("MyOriginalTable") , &vsCols );


    CreateFields( _T("TempTable1") , false );

    tstring sFieldNamesSeperatedByCommas;

    for ( int nCol = 0 ; nCol < vsCols.size() ; nCol++ )
    {

        tstring sColNameCurr = vsCols.at(nCol);

        bool bUseCol = true;

        for ( int nColsToDelete = 0; nColsToDelete < pvsColsToDelete->size() ; nColsToDelete++ )
        {
            if ( pvsColsToDelete->at(nColsToDelete) == sColNameCurr )
            {
                bUseCol = false;
                break;
            }
        }

        if ( bUseCol )
            sFieldNamesSeperatedByCommas+= (sColNameCurr + _T(","));

    }

    if ( sFieldNamesSeperatedByCommas.at( int(sFieldNamesSeperatedByCommas.size()) - 1) == _T(','))
        sFieldNamesSeperatedByCommas.erase( int(sFieldNamesSeperatedByCommas.size()) - 1 );

    tstring sDML;


    sDML = StringOps::std_sprintf(_T("insert into TempTable1 SELECT %s FROM MyOriginalTable;\n") , sFieldNamesSeperatedByCommas.c_str() );
    execDML( StringOps::tstringTo_stdString(sDML).c_str() );

    sDML = StringOps::std_sprintf(_T("ALTER TABLE MyOriginalTable RENAME TO MyOriginalTable_old\n") );
    execDML( StringOps::tstringTo_stdString(sDML).c_str() );

    sDML = StringOps::std_sprintf(_T("ALTER TABLE TempTable1 RENAME TO MyOriginalTable\n") );
    execDML( StringOps::tstringTo_stdString(sDML).c_str() );


    sDML = ( _T("DROP TABLE MyOriginalTable_old;") );   
    execDML( StringOps::tstringTo_stdString(sDML).c_str() );


    execDML( StringOps::tstringTo_stdString(_T("commit transaction;")).c_str() );   
}

答案 8 :(得分：1)

我制作了一个 Python 函数，您可以在其中输入要删除的表和列作为参数：

def removeColumn(table, column):
    columns = []
    for row in c.execute('PRAGMA table_info(' + table + ')'):
        columns.append(row[1])
    columns.remove(column)
    columns = str(columns)
    columns = columns.replace("[", "(")
    columns = columns.replace("]", ")")
    for i in ["\'", "(", ")"]:
        columns = columns.replace(i, "")
    c.execute('CREATE TABLE temptable AS SELECT ' + columns + ' FROM ' + table)
    c.execute('DROP TABLE ' + table)
    c.execute('ALTER TABLE temptable RENAME TO ' + table)
    conn.commit()

根据Duda和MeBigFatGuy回答的信息，如果表上有外键，则此方法将不起作用，但是可以用两行代码解决此问题（创建新表，而不仅仅是重命名临时表）

答案 9 :(得分：0)

如果有人需要（几乎）即用型PHP函数，以下是based on this answer：

/**
 * Remove a column from a table.
 * 
 * @param string $tableName The table to remove the column from.
 * @param string $columnName The column to remove from the table.
 */
public function DropTableColumn($tableName, $columnName)
{
    // --
    // Determine all columns except the one to remove.

    $columnNames = array();

    $statement = $pdo->prepare("PRAGMA table_info($tableName);");
    $statement->execute(array());
    $rows = $statement->fetchAll(PDO::FETCH_OBJ);

    $hasColumn = false;

    foreach ($rows as $row)
    {
        if(strtolower($row->name) !== strtolower($columnName))
        {
            array_push($columnNames, $row->name);
        }
        else
        {
            $hasColumn = true;
        }
    }

    // Column does not exist in table, no need to do anything.
    if ( !$hasColumn ) return;

    // --
    // Actually execute the SQL.

    $columns = implode('`,`', $columnNames);

    $statement = $pdo->exec(
       "CREATE TABLE `t1_backup` AS SELECT `$columns` FROM `$tableName`;
        DROP TABLE `$tableName`;
        ALTER TABLE `t1_backup` RENAME TO `$tableName`;");
}

与其他答案相比，此方法中使用的SQL似乎保留了列的数据类型，而类似接受的答案似乎导致所有列都为TEXT类型。

更新1：

使用的SQL有一个缺点，autoincrement列不保留。

答案 10 :(得分：0)

以防万一它可以帮助像我这样的人。

基于Official website和Accepted answer，我使用 C＃编写了代码，该代码使用了 System.Data.SQLite NuGet包。< / p>

此代码还保留了主键和外键。

C＃中的代码： 

void RemoveColumnFromSqlite (string tableName, string columnToRemove) {
 try {
    var mSqliteDbConnection = new SQLiteConnection ("Data Source=db_folder\\MySqliteBasedApp.db;Version=3;Page Size=1024;");
    mSqliteDbConnection.Open ();             
    // Reads all columns definitions from table
    List<string> columnDefinition = new List<string> ();
    var mSql = $"SELECT type, sql FROM sqlite_master WHERE tbl_name='{tableName}'";
    var mSqliteCommand = new SQLiteCommand (mSql, mSqliteDbConnection);
    string sqlScript = "";
    using (mSqliteReader = mSqliteCommand.ExecuteReader ()) {
       while (mSqliteReader.Read ()) {
          sqlScript = mSqliteReader["sql"].ToString ();
          break;
       }
    }
    if (!string.IsNullOrEmpty (sqlScript)) {
       // Gets string within first '(' and last ')' characters
       int firstIndex = sqlScript.IndexOf ("(");
       int lastIndex = sqlScript.LastIndexOf (")");
       if (firstIndex >= 0 && lastIndex <= sqlScript.Length - 1) {
          sqlScript = sqlScript.Substring (firstIndex, lastIndex - firstIndex + 1);
       }
       string[] scriptParts = sqlScript.Split (new string[] { "," }, StringSplitOptions.RemoveEmptyEntries);
       foreach (string s in scriptParts) {
          if (!s.Contains (columnToRemove)) {
             columnDefinition.Add (s);
          }
       }
    }
    string columnDefinitionString = string.Join (",", columnDefinition);
    // Reads all columns from table
    List<string> columns = new List<string> ();
    mSql = $"PRAGMA table_info({tableName})";
    mSqliteCommand = new SQLiteCommand (mSql, mSqliteDbConnection);
    using (mSqliteReader = mSqliteCommand.ExecuteReader ()) {
       while (mSqliteReader.Read ()) columns.Add (mSqliteReader["name"].ToString ());
    }
    columns.Remove (columnToRemove);
    string columnString = string.Join (",", columns);
    mSql = "PRAGMA foreign_keys=OFF";
    mSqliteCommand = new SQLiteCommand (mSql, mSqliteDbConnection);
    int n = mSqliteCommand.ExecuteNonQuery ();
    // Removes a column from the table
    using (SQLiteTransaction tr = mSqliteDbConnection.BeginTransaction ()) {
       using (SQLiteCommand cmd = mSqliteDbConnection.CreateCommand ()) {
          cmd.Transaction = tr;
          string query = $"CREATE TEMPORARY TABLE {tableName}_backup {columnDefinitionString}";
          cmd.CommandText = query;
          cmd.ExecuteNonQuery ();
          cmd.CommandText = $"INSERT INTO {tableName}_backup SELECT {columnString} FROM {tableName}";
          cmd.ExecuteNonQuery ();
          cmd.CommandText = $"DROP TABLE {tableName}";
          cmd.ExecuteNonQuery ();
          cmd.CommandText = $"CREATE TABLE {tableName} {columnDefinitionString}";
          cmd.ExecuteNonQuery ();
          cmd.CommandText = $"INSERT INTO {tableName} SELECT {columnString} FROM {tableName}_backup;";
          cmd.ExecuteNonQuery ();
          cmd.CommandText = $"DROP TABLE {tableName}_backup";
          cmd.ExecuteNonQuery ();
       }
       tr.Commit ();
    }
    mSql = "PRAGMA foreign_keys=ON";
    mSqliteCommand = new SQLiteCommand (mSql, mSqliteDbConnection);
    n = mSqliteCommand.ExecuteNonQuery ();
 } catch (Exception ex) {
    HandleExceptions (ex);
 }
}

答案 11 :(得分：0)

在Python 3.8中... 保留主键和列类型。

接受3个输入：

  1. sqlite游标：db_cur，
  2. 表名：t和
  3. 要丢弃的列的列表：columns_to_junk 
def removeColumns(db_cur, t, columns_to_junk):

    # Obtain column information
    sql = "PRAGMA table_info(" + t + ")"
    record = query(db_cur, sql)

    # Initialize two strings: one for column names + column types and one just
    # for column names
    cols_w_types = "("
    cols = ""

    # Build the strings, filtering for the column to throw out
    for r in record:
        if r[1] not in columns_to_junk:
            if r[5] == 0:
                cols_w_types += r[1] + " " + r[2] + ","
            if r[5] == 1:
                cols_w_types += r[1] + " " + r[2] + " PRIMARY KEY,"
            cols += r[1] + ","

    # Cut potentially trailing commas
    if cols_w_types[-1] == ",":
        cols_w_types = cols_w_types[:-1]
    else:
        pass

    if cols[-1] == ",":
        cols = cols[:-1]
    else:
        pass

    # Execute SQL
    sql = "CREATE TEMPORARY TABLE xfer " + cols_w_types + ")"
    db_cur.execute(sql)
    sql = "INSERT INTO xfer SELECT " + cols + " FROM " + t
    db_cur.execute(sql)
    sql = "DROP TABLE " + t
    db_cur.execute(sql)
    sql = "CREATE TABLE " + t + cols_w_types + ")"
    db_cur.execute(sql)
    sql = "INSERT INTO " + t + " SELECT " + cols  + " FROM xfer"
    db_cur.execute(sql)

您将找到对query（）函数的引用。只是一个帮手...

接受两个输入：

  1. sqlite游标db_cur和
  2. 查询字符串：query 
def query(db_cur, query):

    r = db_cur.execute(query).fetchall()

    return r

别忘了添加一个“ commit（）”！

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