** If rMayHaveNull is non-zero, that means that the operation is an IN
** (not a SELECT or EXISTS) and that the RHS might contains NULLs.
** All this routine does is initialize the register given by rMayHaveNull
** to NULL.  Calling routines will take care of changing this register
** value to non-NULL if the RHS is NULL-free.
**
** For a SELECT or EXISTS operator, return the register that holds the


** result.  For IN operators or if an error occurs, the return value is 0.
*/
#ifndef SQLITE_OMIT_SUBQUERY
int sqlite3CodeSubselect(
  Parse *pParse,          /* Parsing context */
  Expr *pExpr,            /* The IN, SELECT, or EXISTS operator */
  int rHasNullFlag,       /* Register that records whether NULLs exist in RHS */
  int isRowid             /* If true, LHS of IN operator is a rowid */
................................................................................
){
  int jmpIfDynamic = -1;                      /* One-time test address */
  int rReg = 0;                           /* Register storing resulting */
  Vdbe *v = sqlite3GetVdbe(pParse);
  if( NEVER(v==0) ) return 0;
  sqlite3ExprCachePush(pParse);

  /* This code must be run in its entirety every time it is encountered
  ** if any of the following is true:
  **
  **    *  The right-hand side is a correlated subquery
  **    *  The right-hand side is an expression list containing variables
  **    *  We are inside a trigger
  **
  ** If all of the above are false, then we can run this code just once
  ** save the results, and reuse the same result on subsequent invocations.
................................................................................
      }
      break;
    }

    case TK_EXISTS:
    case TK_SELECT:
    default: {
      /* If this has to be a scalar SELECT.  Generate code to put the
      ** value of this select in a memory cell and record the number
      ** of the memory cell in iColumn.  If this is an EXISTS, write




      ** an integer 0 (not exists) or 1 (exists) into a memory cell
      ** and record that memory cell in iColumn.




      */
      Select *pSel;                         /* SELECT statement to encode */
      SelectDest dest;                      /* How to deal with SELECt result */
      int nReg;                             /* Registers to allocate */

      testcase( pExpr->op==TK_EXISTS );
      testcase( pExpr->op==TK_SELECT );
      assert( pExpr->op==TK_EXISTS || pExpr->op==TK_SELECT );
      assert( ExprHasProperty(pExpr, EP_xIsSelect) );

** If rMayHaveNull is non-zero, that means that the operation is an IN
** (not a SELECT or EXISTS) and that the RHS might contains NULLs.
** All this routine does is initialize the register given by rMayHaveNull
** to NULL.  Calling routines will take care of changing this register
** value to non-NULL if the RHS is NULL-free.
**
** For a SELECT or EXISTS operator, return the register that holds the
** result.  For a multi-column SELECT, the result is stored in a contiguous
** array of registers and the return value is the register of the left-most
** result column.  Return 0 for IN operators or if an error occurs.
*/
#ifndef SQLITE_OMIT_SUBQUERY
int sqlite3CodeSubselect(
  Parse *pParse,          /* Parsing context */
  Expr *pExpr,            /* The IN, SELECT, or EXISTS operator */
  int rHasNullFlag,       /* Register that records whether NULLs exist in RHS */
  int isRowid             /* If true, LHS of IN operator is a rowid */
................................................................................
){
  int jmpIfDynamic = -1;                      /* One-time test address */
  int rReg = 0;                           /* Register storing resulting */
  Vdbe *v = sqlite3GetVdbe(pParse);
  if( NEVER(v==0) ) return 0;
  sqlite3ExprCachePush(pParse);

  /* The evaluation of the IN/EXISTS/SELECT must be repeated every time it
  ** is encountered if any of the following is true:
  **
  **    *  The right-hand side is a correlated subquery
  **    *  The right-hand side is an expression list containing variables
  **    *  We are inside a trigger
  **
  ** If all of the above are false, then we can run this code just once
  ** save the results, and reuse the same result on subsequent invocations.
................................................................................
      }
      break;
    }

    case TK_EXISTS:
    case TK_SELECT:
    default: {
      /* Case 3:    (SELECT ... FROM ...)
      **     or:    EXISTS(SELECT ... FROM ...)
      **
      ** For a SELECT, generate code to put the values for all columns of
      ** the first row into an array of registers and return the index of
      ** the first register.
      **
      ** If this is an EXISTS, write an integer 0 (not exists) or 1 (exists)

      ** into a register and return that register number.
      **
      ** In both cases, the query is augmented with "LIMIT 1".  Any 
      ** preexisting limit is discarded in place of the new LIMIT 1.
      */
      Select *pSel;                         /* SELECT statement to encode */
      SelectDest dest;                      /* How to deal with SELECT result */
      int nReg;                             /* Registers to allocate */

      testcase( pExpr->op==TK_EXISTS );
      testcase( pExpr->op==TK_SELECT );
      assert( pExpr->op==TK_EXISTS || pExpr->op==TK_SELECT );
      assert( ExprHasProperty(pExpr, EP_xIsSelect) );