C# Expr.CopyFrom方法代码示例

//.........这里部分代码省略.........
    **  C#-SQLite is an independent reimplementation of the SQLite software library
    **
    **  SQLITE_SOURCE_ID: 2010-08-23 18:52:01 42537b60566f288167f1b5864a5435986838e3a3
    **
    *************************************************************************
    */
    //#include "sqliteInt.h"
    //#include <stdlib.h>
    //#include <string.h>

    /*
    ** Turn the pExpr expression into an alias for the iCol-th column of the
    ** result set in pEList.
    **
    ** If the result set column is a simple column reference, then this routine
    ** makes an exact copy.  But for any other kind of expression, this
    ** routine make a copy of the result set column as the argument to the
    ** TK_AS operator.  The TK_AS operator causes the expression to be
    ** evaluated just once and then reused for each alias.
    **
    ** The reason for suppressing the TK_AS term when the expression is a simple
    ** column reference is so that the column reference will be recognized as
    ** usable by indices within the WHERE clause processing logic.
    **
    ** Hack:  The TK_AS operator is inhibited if zType[0]=='G'.  This means
    ** that in a GROUP BY clause, the expression is evaluated twice.  Hence:
    **
    **     SELECT random()%5 AS x, count(*) FROM tab GROUP BY x
    **
    ** Is equivalent to:
    **
    **     SELECT random()%5 AS x, count(*) FROM tab GROUP BY random()%5
    **
    ** The result of random()%5 in the GROUP BY clause is probably different
    ** from the result in the result-set.  We might fix this someday.  Or
    ** then again, we might not...
    */
    static void resolveAlias(
    Parse pParse,         /* Parsing context */
    ExprList pEList,      /* A result set */
    int iCol,             /* A column in the result set.  0..pEList.nExpr-1 */
    Expr pExpr,       /* Transform this into an alias to the result set */
    string zType          /* "GROUP" or "ORDER" or "" */
    )
    {
      Expr pOrig;           /* The iCol-th column of the result set */
      Expr pDup;            /* Copy of pOrig */
      sqlite3 db;           /* The database connection */

      Debug.Assert( iCol >= 0 && iCol < pEList.nExpr );
      pOrig = pEList.a[iCol].pExpr;
      Debug.Assert( pOrig != null );
      Debug.Assert( ( pOrig.flags & EP_Resolved ) != 0 );
      db = pParse.db;
      if ( pOrig.op != TK_COLUMN && ( zType.Length == 0 || zType[0] != 'G' ) )
      {
        pDup = sqlite3ExprDup( db, pOrig, 0 );
        pDup = sqlite3PExpr( pParse, TK_AS, pDup, null, null );
        if ( pDup == null )
          return;
        if ( pEList.a[iCol].iAlias == 0 )
        {
          pEList.a[iCol].iAlias = (u16)( ++pParse.nAlias );
        }
        pDup.iTable = pEList.a[iCol].iAlias;
      }
      else if ( ExprHasProperty( pOrig, EP_IntValue ) || pOrig.u.zToken == null )
      {
        pDup = sqlite3ExprDup( db, pOrig, 0 );
        if ( pDup == null )
          return;
      }
      else
      {
        string zToken = pOrig.u.zToken;
        Debug.Assert( zToken != null );
        pOrig.u.zToken = null;
        pDup = sqlite3ExprDup( db, pOrig, 0 );
        pOrig.u.zToken = zToken;
        if ( pDup == null )
          return;
        Debug.Assert( ( pDup.flags & ( EP_Reduced | EP_TokenOnly ) ) == 0 );
        pDup.flags2 |= EP2_MallocedToken;
        pDup.u.zToken = zToken;// sqlite3DbStrDup( db, zToken );
      }
      if ( ( pExpr.flags & EP_ExpCollate ) != 0 )
      {
        pDup.pColl = pExpr.pColl;
        pDup.flags |= EP_ExpCollate;
      }

      /* Before calling sqlite3ExprDelete(), set the EP_Static flag. This 
      ** prevents ExprDelete() from deleting the Expr structure itself,
      ** allowing it to be repopulated by the memcpy() on the following line.
      */
      ExprSetProperty( pExpr, EP_Static );
      sqlite3ExprDelete( db, ref pExpr );
      pExpr.CopyFrom( pDup ); //memcpy(pExpr, pDup, sizeof(*pExpr));
      sqlite3DbFree( db, ref pDup );
    }