SQLite

**
** All references to columns in table iTable are to be replaced by corresponding
** expressions in pEList.
*/
typedef struct SubstContext {
  Parse *pParse;            /* The parsing context */
  int iTable;               /* Replace references to this table */
  int iNewTable;            /* New table number, or -1 */
  ExprList *pEList;         /* Replacement expressions */
} SubstContext;

/* Forward Declarations */
static void substExprList(SubstContext*, ExprList*);
static void substSelect(SubstContext*, Select*, int);

** of the subquery rather the result set of the subquery.
*/
static Expr *substExpr(
  SubstContext *pSubst,  /* Description of the substitution */
  Expr *pExpr            /* Expr in which substitution occurs */
){
  if( pExpr==0 ) return 0;
  if( ExprHasProperty(pExpr, EP_FromJoin) && pExpr->iRightJoinTable==pSubst->iTable ){
    pExpr->iRightJoinTable = pSubst->iNewTable;
  }
  if( pExpr->op==TK_COLUMN && pExpr->iTable==pSubst->iTable ){
    if( pExpr->iColumn<0 ){
      pExpr->op = TK_NULL;
    }else{
      Expr *pNew;
      Expr *pCopy = pSubst->pEList->a[pExpr->iColumn].pExpr;
      assert( pSubst->pEList!=0 && pExpr->iColumn<pSubst->pEList->nExpr );

**
**   (2)  The subquery is not an aggregate or (2a) the outer query is not a join
**        and (2b) the outer query does not use subqueries other than the one
**        FROM-clause subquery that is a candidate for flattening.  (2b is
**        due to ticket [2f7170d73bf9abf80] from 2015-02-09.)
**
**   (3)  The subquery is not the right operand of a left outer join
**        or (3b) the subquery is not a join and (3c) does not contain any
**        result-set columns that could be non-NULL even if columns of the
**        subquery table are NULL.
**        to be more restrictive - disallowing subquery if it was the
**        right operand of a left join.  See tickets #306 and #3300. Relaxed
**        to allow simple subqueries as the right operand of a left join
**        on 2017-04-14.)
**
**   (4)  The subquery is not DISTINCT.
**
**  (**)  At one point restrictions (4) and (5) defined a subset of DISTINCT
**        sub-queries that were excluded from this optimization. Restriction 
**        (4) has since been expanded to exclude all DISTINCT subqueries.
**

  Select *pParent;    /* Current UNION ALL term of the other query */
  Select *pSub;       /* The inner query or "subquery" */
  Select *pSub1;      /* Pointer to the rightmost select in sub-query */
  SrcList *pSrc;      /* The FROM clause of the outer query */
  SrcList *pSubSrc;   /* The FROM clause of the subquery */
  ExprList *pList;    /* The result set of the outer query */
  int iParent;        /* VDBE cursor number of the pSub result set temp table */
  int iNewParent = -1;/* Replacement table for iParent */
  int isLeftJoin = 0; /* True if pSub is the right side of a LEFT JOIN */    
  int i;              /* Loop counter */
  Expr *pWhere;                    /* The WHERE clause */
  struct SrcList_item *pSubitem;   /* The subquery */
  sqlite3 *db = pParse->db;

  /* Check to see if flattening is permitted.  Return 0 if not.
  */

    if( (p->pWhere && ExprHasProperty(p->pWhere,EP_Subquery))
     || (sqlite3ExprListFlags(p->pEList) & EP_Subquery)!=0
     || (sqlite3ExprListFlags(p->pOrderBy) & EP_Subquery)!=0
    ){
      return 0;                                          /* Restriction (2b)  */
    }
  }

  pSubSrc = pSub->pSrc;
  assert( pSubSrc );
  /* Prior to version 3.1.2, when LIMIT and OFFSET had to be simple constants,
  ** not arbitrary expressions, 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->selFlags & (SF_Recursive|SF_MinMaxAgg) ){
    return 0; /* Restrictions (22) and (24) */
  }
  if( (p->selFlags & SF_Recursive) && pSub->pPrior ){
    return 0; /* Restriction (23) */
  }

  /*
  ** If the subquery is the right operand of a LEFT JOIN, then the
  ** subquery may not be a join itself.  Example of why this is not allowed:

  **
  **         t1 LEFT OUTER JOIN (t2 JOIN t3)
  **
  ** If we flatten the above, we would get
  **
  **         (t1 LEFT OUTER JOIN t2) JOIN t3
  **
  ** which is not at all the same thing.
  **
  ** See also tickets #306 and #3300.

















  */
  if( (pSubitem->fg.jointype & JT_OUTER)!=0 ){
    isLeftJoin = 1;
    if( pSubSrc->nSrc>1 ){
      return 0; /* Restriction (3b) */
    }
    /* TBD: Deal with the case of result-set expressions that are non-NULL even
    ** when the RHS table of a LEFT JOIN is NULL. */
  }

  /* Restriction 17: If the sub-query is a compound SELECT, then it must
  ** use only the UNION ALL operator. And none of the simple select queries
  ** that make up the compound SELECT are allowed to be aggregate or distinct
  ** queries.
  */

    /* Transfer the FROM clause terms from the subquery into the
    ** outer query.
    */
    for(i=0; i<nSubSrc; i++){
      sqlite3IdListDelete(db, pSrc->a[i+iFrom].pUsing);
      assert( pSrc->a[i+iFrom].fg.isTabFunc==0 );
      pSrc->a[i+iFrom] = pSubSrc->a[i];
      iNewParent = pSubSrc->a[i].iCursor;
      memset(&pSubSrc->a[i], 0, sizeof(pSubSrc->a[i]));
    }
    pSrc->a[iFrom].fg.jointype = jointype;
  
    /* Now begin substituting subquery result set expressions for 
    ** references to the iParent in the outer query.
    ** 

      }
      assert( pParent->pOrderBy==0 );
      assert( pSub->pPrior==0 );
      pParent->pOrderBy = pOrderBy;
      pSub->pOrderBy = 0;
    }
    pWhere = sqlite3ExprDup(db, pSub->pWhere, 0);
    if( isLeftJoin ){
      setJoinExpr(pWhere, iNewParent);
    }
    if( subqueryIsAgg ){
      assert( pParent->pHaving==0 );
      pParent->pHaving = pParent->pWhere;
      pParent->pWhere = pWhere;
      pParent->pHaving = sqlite3ExprAnd(db, 
          sqlite3ExprDup(db, pSub->pHaving, 0), pParent->pHaving
      );
      assert( pParent->pGroupBy==0 );
      pParent->pGroupBy = sqlite3ExprListDup(db, pSub->pGroupBy, 0);
    }else{
      pParent->pWhere = sqlite3ExprAnd(db, pWhere, pParent->pWhere);
    }
    if( db->mallocFailed==0 ){
      SubstContext x;
      x.pParse = pParse;
      x.iTable = iParent;
      x.iNewTable = iNewParent;
      x.pEList = pSub->pEList;
      substSelect(&x, pParent, 0);
    }
  
    /* The flattened query is distinct if either the inner or the
    ** outer query is distinct. 
    */

  if( sqlite3ExprIsTableConstant(pWhere, iCursor) ){
    nChng++;
    while( pSubq ){
      SubstContext x;
      pNew = sqlite3ExprDup(pParse->db, pWhere, 0);
      x.pParse = pParse;
      x.iTable = iCursor;
      x.iNewTable = iCursor;
      x.pEList = pSubq->pEList;
      pNew = substExpr(&x, pNew);
      pSubq->pWhere = sqlite3ExprAnd(pParse->db, pSubq->pWhere, pNew);
      pSubq = pSubq->pPrior;
    }
  }
  return nChng;