关于sql：从SQLite表中删除列

Delete column from SQLite table

我有一个问题：我需要从SQLite数据库中删除一列。我写了这个查询

1	ALTER TABLE TABLE_NAME DROP COLUMN column_name

但它不起作用。请帮我。

来自：http://www.sqlite.org/faq.html：

(11) How do I add or delete columns from an existing table in SQLite.

SQLite has limited ALTER TABLE support that you can use to add a
column to the end of a table or to change the name of a table. If you
want to make more complex changes in the structure of a table, you
will have to recreate the table. You can save existing data to a
temporary table, drop the old table, create the new table, then copy
the data back in from the temporary table.

For example, suppose you have a table named"t1" with columns names
"a","b", and"c" and that you want to delete column"c" from this
table. The following steps illustrate how this could be done:

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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;

相关讨论

仅当您可以在数据库浏览器(如用于SQLite的数据库浏览器)中打开数据库时，此选项才有效。

在SQLite的数据库浏览器中：

转到标签"数据库结构"

选择您的表选择"修改"表(在选项卡下)

选择要删除的列

单击删除字段，然后单击确定

=>直接使用以下查询创建新表：

1	CREATE TABLE TABLE_NAME (Column_1 TEXT,Column_2 TEXT);

=>现在使用以下查询将数据从existing_table插入table_name中：

1	INSERT INTO TABLE_NAME (Column_1,Column_2) FROM existing_table;

=>现在通过以下查询删除现存表：

1	DROP TABLE existing_table;

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：

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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的答案的信息，如果表上有外键，这将不起作用，但是可以用两行代码来解决(创建新表，而不仅仅是重命名临时表)

对于SQLite3 c ++：

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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;\
") , sFieldNamesSeperatedByCommas.c_str() );
execDML( StringOps::tstringTo_stdString(sDML).c_str() );

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

sDML = StringOps::std_sprintf(_T("ALTER TABLE TempTable1 RENAME TO MyOriginalTable\
") );
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() );
}

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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

以防万一它可以帮助像我这样的人。

基于官方网站和"已接受"的答案，我使用C＃编写了一个代码，该代码使用了System.Data.SQLite NuGet包。

此代码还保留了主键和外键。

C＃中的代码：

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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);
}
}

如果有人需要(几乎)立即可用的PHP函数，则基于此答案的内容如下：

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/**
* 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`;");
}

与其他答案相反，此方法中使用的SQL似乎保留了列的数据类型，而类似的答案似乎导致所有列的类型均为TEXT。

更新1：

使用的SQL的缺点是不会保留autoincrement列。