SQLite

**    May you find forgiveness for yourself and forgive others.
**    May you share freely, never taking more than you give.
**
*************************************************************************
** This file contains C code routines that are called by the parser
** to handle SELECT statements in SQLite.
**
** $Id: select.c,v 1.297 2006/01/21 22:19:55 drh Exp $
*/
#include "sqliteInt.h"


/*
** Allocate a new Select structure and return a pointer to that
** structure.

**  (10)  The subquery does not use aggregates or the outer query does not
**        use LIMIT.
**
**  (11)  The subquery and the outer query do not both have ORDER BY clauses.
**
**  (12)  The subquery is not the right term of a LEFT OUTER JOIN or the
**        subquery has no WHERE clause.  (added by ticket #350)
**
**  (13)  The subquery and outer query do not both use LIMIT
**
**  (14)  The subquery does not use OFFSET
**
** In this routine, the "p" parameter is a pointer to the outer query.
** The subquery is p->pSrc->a[iFrom].  isAgg is true if the outer query
** uses aggregates and subqueryIsAgg is true if the subquery uses aggregates.
**
** If flattening is not attempted, this routine is a no-op and returns 0.
** If flattening is attempted this routine returns 1.

  */
  if( p==0 ) return 0;
  pSrc = p->pSrc;
  assert( pSrc && iFrom>=0 && iFrom<pSrc->nSrc );
  pSubitem = &pSrc->a[iFrom];
  pSub = pSubitem->pSelect;
  assert( pSub!=0 );
  if( isAgg && subqueryIsAgg ) return 0;                 /* Restriction (1)  */
  if( subqueryIsAgg && pSrc->nSrc>1 ) return 0;          /* Restriction (2)  */
  pSubSrc = pSub->pSrc;
  assert( pSubSrc );
  /* Prior to version 3.1.2, when LIMIT and OFFSET had to be simple constants,
  ** not arbitrary expresssions, we allowed some combining of LIMIT and OFFSET
  ** because they could be computed at compile-time.  But when LIMIT and OFFSET
  ** became arbitrary expressions, we were forced to add restrictions (13)
  ** and (14). */
  if( pSub->pLimit && p->pLimit ) return 0;              /* Restriction (13) */
  if( pSub->pOffset ) return 0;                          /* Restriction (14) */
  if( pSubSrc->nSrc==0 ) return 0;                       /* Restriction (7)  */
  if( (pSub->isDistinct || pSub->pLimit) 
         && (pSrc->nSrc>1 || isAgg) ){          /* Restrictions (4)(5)(8)(9) */
     return 0;       
  }
  if( p->isDistinct && subqueryIsAgg ) return 0;         /* Restriction (6)  */
  if( (p->disallowOrderBy || p->pOrderBy) && pSub->pOrderBy ){
     return 0;                                           /* Restriction (11) */
  }

  /* Restriction 3:  If the subquery is a join, make sure the subquery is 
  ** not used as the right operand of an outer join.  Examples of why this
  ** is not allowed:
  **
  **         t1 LEFT OUTER JOIN (t2 JOIN t3)
  **

  /* The flattened query is distinct if either the inner or the
  ** outer query is distinct. 
  */
  p->isDistinct = p->isDistinct || pSub->isDistinct;

  /*
  ** SELECT ... FROM (SELECT ... LIMIT a OFFSET b) LIMIT x OFFSET y;
  **
  ** One is tempted to try to add a and b to combine the limits.  But this
  ** does not work if either limit is negative.
  */
  if( pSub->pLimit ){
    p->pLimit = pSub->pLimit;
    pSub->pLimit = 0;
  }

  /* Finially, delete what is left of the subquery and return