** in SQL, this function assumes that the input is a well-formed quoted SQL 
** string. In this case the string is dequoted in place.
**
** If the first character of the input is not an open quote, then this
** function is a no-op.
*/
static void unionDequote(char *z){

  char q = z[0];

  /* Set stack variable q to the close-quote character */
  if( q=='[' || q=='\'' || q=='"' || q=='`' ){
    int iIn = 1;
    int iOut = 0;
    if( q=='[' ) q = ']';  
    while( z[iIn] ){
      if( z[iIn]==q ){
        if( z[iIn+1]!=q ){
          /* Character iIn was the close quote. */
          iIn++;
          break;
        }else{
          /* Character iIn and iIn+1 form an escaped quote character. Skip
          ** the input cursor past both and copy a single quote character 
          ** to the output buffer. */
          iIn += 2;
          z[iOut++] = q;
        }
      }else{
        z[iOut++] = z[iIn++];
      }
    }
    z[iOut] = '\0';

  }
}

/*
** This function is a no-op if *pRc is set to other than SQLITE_OK when it
** is called. NULL is returned in this case.
**
................................................................................
    if( rc!=SQLITE_OK ){
      *pzErr = sqlite3_mprintf("sql error: %s", sqlite3_errmsg(db));
      *pRc = rc;
    }
  }
  return pRet;
}
































/*
** Call sqlite3_reset() on SQL statement pStmt. If *pRc is set to 
** SQLITE_OK when this function is called, then it is set to the
** value returned by sqlite3_reset() before this function exits.
** In this case, *pzErr may be set to point to an error message
** buffer allocated by sqlite3_malloc().
................................................................................
    char *zSql = 0;               /* SQL statement */
    char *zArg = unionStrdup(&rc, argv[3]);      /* Copy of argument to CVT */

    /* Prepare the SQL statement. Instead of executing it directly, sort
    ** the results by the "minimum rowid" field. This makes it easier to
    ** check that there are no rowid range overlaps between source tables 
    ** and that the UnionTab.aSrc[] array is always sorted by rowid.  */
    if( zArg ){
      unionDequote(zArg);

      zSql = sqlite3_mprintf("SELECT * FROM (%s) ORDER BY 3", zArg);
      sqlite3_free(zArg);
      zArg = 0;
    }
    if( zSql==0 ){
      rc = SQLITE_NOMEM;
    }
    pStmt = unionPrepare(&rc, db, zSql, pzErr);

    /* Allocate the UnionTab structure */
    pTab = unionMalloc(&rc, sizeof(UnionTab));

    /* Iterate through the rows returned by the SQL statement specified
    ** as an argument to the CREATE VIRTUAL TABLE statement. */
    while( rc==SQLITE_OK && SQLITE_ROW==sqlite3_step(pStmt) ){
................................................................................
      pSrc->zDb = unionStrdup(&rc, zDb);
      pSrc->zTab = unionStrdup(&rc, zTab);
      pSrc->iMin = iMin;
      pSrc->iMax = iMax;
    }
    unionFinalize(&rc, pStmt);
    pStmt = 0;
    sqlite3_free(zArg);
    sqlite3_free(zSql);
    zArg = 0;
    zSql = 0;

    /* Verify that all source tables exist and have compatible schemas. */
    if( rc==SQLITE_OK ){
      pTab->db = db;
      rc = unionSourceCheck(pTab, pzErr);
    }

    /* Compose a CREATE TABLE statement and pass it to declare_vtab() */
    if( rc==SQLITE_OK ){
      zSql = sqlite3_mprintf("SELECT "
          "'CREATE TABLE xyz('"
          "    || group_concat(quote(name) || ' ' || type, ', ')"
          "    || ')',"
          "max((cid+1) * (type='INTEGER' COLLATE nocase AND pk=1))-1 "
          "FROM pragma_table_info(%Q, ?)", 
          pTab->aSrc[0].zTab, pTab->aSrc[0].zDb
      );
      if( zSql==0 ) rc = SQLITE_NOMEM;
    }
    pStmt = unionPrepare(&rc, db, zSql, pzErr);
    if( rc==SQLITE_OK && SQLITE_ROW==sqlite3_step(pStmt) ){
      const char *zDecl = (const char*)sqlite3_column_text(pStmt, 0);
      rc = sqlite3_declare_vtab(db, zDecl);
      pTab->iPK = sqlite3_column_int(pStmt, 1);
    }

    unionFinalize(&rc, pStmt);

** in SQL, this function assumes that the input is a well-formed quoted SQL 
** string. In this case the string is dequoted in place.
**
** If the first character of the input is not an open quote, then this
** function is a no-op.
*/
static void unionDequote(char *z){
  if( z ){
    char q = z[0];

    /* Set stack variable q to the close-quote character */
    if( q=='[' || q=='\'' || q=='"' || q=='`' ){
      int iIn = 1;
      int iOut = 0;
      if( q=='[' ) q = ']';  
      while( z[iIn] ){
        if( z[iIn]==q ){
          if( z[iIn+1]!=q ){
            /* Character iIn was the close quote. */
            iIn++;
            break;
          }else{
            /* Character iIn and iIn+1 form an escaped quote character. Skip
            ** the input cursor past both and copy a single quote character 
            ** to the output buffer. */
            iIn += 2;
            z[iOut++] = q;
          }
        }else{
          z[iOut++] = z[iIn++];
        }
      }
      z[iOut] = '\0';
    }
  }
}

/*
** This function is a no-op if *pRc is set to other than SQLITE_OK when it
** is called. NULL is returned in this case.
**
................................................................................
    if( rc!=SQLITE_OK ){
      *pzErr = sqlite3_mprintf("sql error: %s", sqlite3_errmsg(db));
      *pRc = rc;
    }
  }
  return pRet;
}

/*
** Like unionPrepare(), except prepare the results of vprintf(zFmt, ...)
** instead of a constant SQL string.
*/
static sqlite3_stmt *unionPreparePrintf(
  int *pRc,                       /* IN/OUT: Error code */
  char **pzErr,                   /* OUT: Error message */
  sqlite3 *db,                    /* Database handle */
  const char *zFmt,               /* printf() format string */
  ...                             /* Trailing printf args */
){
  sqlite3_stmt *pRet = 0;
  char *zSql;
  va_list ap;
  va_start(ap, zFmt);

  zSql = sqlite3_vmprintf(zFmt, ap);
  if( *pRc==SQLITE_OK ){
    if( zSql==0 ){
      *pRc = SQLITE_NOMEM;
    }else{
      pRet = unionPrepare(pRc, db, zSql, pzErr);
    }
  }
  sqlite3_free(zSql);

  va_end(ap);
  return pRet;
}


/*
** Call sqlite3_reset() on SQL statement pStmt. If *pRc is set to 
** SQLITE_OK when this function is called, then it is set to the
** value returned by sqlite3_reset() before this function exits.
** In this case, *pzErr may be set to point to an error message
** buffer allocated by sqlite3_malloc().
................................................................................
    char *zSql = 0;               /* SQL statement */
    char *zArg = unionStrdup(&rc, argv[3]);      /* Copy of argument to CVT */

    /* Prepare the SQL statement. Instead of executing it directly, sort
    ** the results by the "minimum rowid" field. This makes it easier to
    ** check that there are no rowid range overlaps between source tables 
    ** and that the UnionTab.aSrc[] array is always sorted by rowid.  */

    unionDequote(zArg);
    pStmt = unionPreparePrintf(&rc, pzErr, db, 
        "SELECT * FROM (%z) ORDER BY 3", zArg
    );







    /* Allocate the UnionTab structure */
    pTab = unionMalloc(&rc, sizeof(UnionTab));

    /* Iterate through the rows returned by the SQL statement specified
    ** as an argument to the CREATE VIRTUAL TABLE statement. */
    while( rc==SQLITE_OK && SQLITE_ROW==sqlite3_step(pStmt) ){
................................................................................
      pSrc->zDb = unionStrdup(&rc, zDb);
      pSrc->zTab = unionStrdup(&rc, zTab);
      pSrc->iMin = iMin;
      pSrc->iMax = iMax;
    }
    unionFinalize(&rc, pStmt);
    pStmt = 0;

    sqlite3_free(zSql);

    zSql = 0;

    /* Verify that all source tables exist and have compatible schemas. */
    if( rc==SQLITE_OK ){
      pTab->db = db;
      rc = unionSourceCheck(pTab, pzErr);
    }

    /* Compose a CREATE TABLE statement and pass it to declare_vtab() */
    pStmt = unionPreparePrintf(&rc, pzErr, db, "SELECT "

        "'CREATE TABLE xyz('"
        "    || group_concat(quote(name) || ' ' || type, ', ')"
        "    || ')',"
        "max((cid+1) * (type='INTEGER' COLLATE nocase AND pk=1))-1 "
        "FROM pragma_table_info(%Q, ?)", 
        pTab->aSrc[0].zTab, pTab->aSrc[0].zDb
    );



    if( rc==SQLITE_OK && SQLITE_ROW==sqlite3_step(pStmt) ){
      const char *zDecl = (const char*)sqlite3_column_text(pStmt, 0);
      rc = sqlite3_declare_vtab(db, zDecl);
      pTab->iPK = sqlite3_column_int(pStmt, 1);
    }

    unionFinalize(&rc, pStmt);