func (nr *nameResolver) resolveColumnInResultFields(cn *ast.ColumnNameExpr, rfs []*ast.ResultField) bool {
	if cn.Name.Table.L != "" {
		// Skip result fields, resolve the column in table source.
		return false
	}
	var matched *ast.ResultField
	for _, rf := range rfs {
		matchAsName := cn.Name.Name.L == rf.ColumnAsName.L
		matchColumnName := rf.ColumnAsName.L == "" && cn.Name.Name.L == rf.Column.Name.L
		if matchAsName || matchColumnName {
			if rf.Column.Name.L == "" {
				// This is not a real table column, resolve it directly.
				cn.Refer = rf
				return true
			}
			if matched == nil {
				matched = rf
			} else {
				sameColumn := matched.Table.Name.L == rf.Table.Name.L && matched.Column.Name.L == rf.Column.Name.L
				if !sameColumn {
					nr.Err = errors.Errorf("column %s is ambiguous.", cn.Name.Name.O)
					return true
				}
			}
		}
	}
	if matched != nil {
		// Bind column.
		cn.Refer = matched
		return true
	}
	return false
}

func (nr *nameResolver) resolveColumnInResultFields(ctx *resolverContext, cn *ast.ColumnNameExpr, rfs []*ast.ResultField) bool {
	var matched *ast.ResultField
	for _, rf := range rfs {
		if cn.Name.Table.L != "" {
			// Check table name
			if rf.TableAsName.L != "" {
				if cn.Name.Table.L != rf.TableAsName.L {
					continue
				}
			} else if cn.Name.Table.L != rf.Table.Name.L {
				continue
			}
		}
		matchAsName := cn.Name.Name.L == rf.ColumnAsName.L
		var matchColumnName bool
		if ctx.inHaving {
			matchColumnName = cn.Name.Name.L == rf.Column.Name.L
		} else {
			matchColumnName = rf.ColumnAsName.L == "" && cn.Name.Name.L == rf.Column.Name.L
		}
		if matchAsName || matchColumnName {
			if rf.Column.Name.L == "" {
				// This is not a real table column, resolve it directly.
				cn.Refer = rf
				rf.Referenced = true
				return true
			}
			if matched == nil {
				matched = rf
			} else {
				sameColumn := matched.TableName == rf.TableName && matched.Column.Name.L == rf.Column.Name.L
				if !sameColumn {
					nr.Err = errors.Errorf("column %s is ambiguous.", cn.Name.Name.O)
					return true
				}
			}
		}
	}
	if matched != nil {
		// If in GroupBy, we clone the ResultField
		if ctx.inGroupBy || ctx.inHaving || ctx.inOrderBy {
			nf := *matched
			expr := matched.Expr
			if cexpr, ok := expr.(*ast.ColumnNameExpr); ok {
				expr = cexpr.Refer.Expr
			}
			nf.Expr = expr
			matched = &nf
		}
		// Bind column.
		cn.Refer = matched
		matched.Referenced = true
		return true
	}
	return false
}

func (nr *nameResolver) resolveColumnInTableSources(cn *ast.ColumnNameExpr, tableSources []*ast.TableSource) (done bool) {
	var matchedResultField *ast.ResultField
	tableNameL := cn.Name.Table.L
	columnNameL := cn.Name.Name.L
	if tableNameL != "" {
		var matchedTable ast.ResultSetNode
		for _, ts := range tableSources {
			if tableNameL == ts.AsName.L {
				// different table name.
				matchedTable = ts
				break
			} else if ts.AsName.L != "" {
				// Table as name shadows table real name.
				continue
			}
			if tn, ok := ts.Source.(*ast.TableName); ok {
				if cn.Name.Schema.L != "" && cn.Name.Schema.L != tn.Schema.L {
					continue
				}
				if tableNameL == tn.Name.L {
					matchedTable = ts
				}
			}
		}
		if matchedTable != nil {
			resultFields := matchedTable.GetResultFields()
			for _, rf := range resultFields {
				if rf.ColumnAsName.L == columnNameL || rf.Column.Name.L == columnNameL {
					// resolve column.
					matchedResultField = rf
					break
				}
			}
		}
	} else {
		for _, ts := range tableSources {
			rfs := ts.GetResultFields()
			for _, rf := range rfs {
				matchAsName := rf.ColumnAsName.L != "" && rf.ColumnAsName.L == columnNameL
				matchColumnName := rf.ColumnAsName.L == "" && rf.Column.Name.L == columnNameL
				if matchAsName || matchColumnName {
					if matchedResultField != nil {
						nr.Err = errors.Errorf("column %s is ambiguous.", cn.Name.Name.O)
						return true
					}
					matchedResultField = rf
				}
			}
		}
	}
	if matchedResultField != nil {
		// Bind column.
		cn.Refer = matchedResultField
		matchedResultField.Referenced = true
		return true
	}
	return false
}

func (nr *nameResolver) resolveColumnInResultFields(ctx *resolverContext, cn *ast.ColumnNameExpr, rfs []*ast.ResultField) bool {
	var matched *ast.ResultField
	for _, rf := range rfs {
		if cn.Name.Table.L != "" {
			// Check table name
			if rf.TableAsName.L != "" {
				if cn.Name.Table.L != rf.TableAsName.L {
					continue
				}
			} else if cn.Name.Table.L != rf.Table.Name.L {
				continue
			}
		}
		matchAsName := cn.Name.Name.L == rf.ColumnAsName.L
		var matchColumnName bool
		if ctx.inHaving {
			matchColumnName = cn.Name.Name.L == rf.Column.Name.L
		} else {
			matchColumnName = rf.ColumnAsName.L == "" && cn.Name.Name.L == rf.Column.Name.L
		}
		if matchAsName || matchColumnName {
			if rf.Column.Name.L == "" {
				// This is not a real table column, resolve it directly.
				cn.Refer = rf
				return true
			}
			if matched == nil {
				matched = rf
			} else {
				sameColumn := matched.TableName == rf.TableName && matched.Column.Name.L == rf.Column.Name.L
				if !sameColumn {
					nr.Err = errors.Errorf("column %s is ambiguous.", cn.Name.Name.O)
					return true
				}
			}
		}
	}
	if matched != nil {
		// Bind column.
		cn.Refer = matched
		return true
	}
	return false
}

func (e *Evaluator) columnName(v *ast.ColumnNameExpr) bool {
	v.SetDatum(*v.Refer.Expr.GetDatum())
	return true
}

// resolveColumnNameInContext looks up and sets ResultField for a column with the ctx.
func (nr *nameResolver) resolveColumnNameInContext(ctx *resolverContext, cn *ast.ColumnNameExpr) bool {
	if ctx.inTableRefs {
		// In TableRefsClause, column reference only in join on condition which is handled before.
		return false
	}
	if ctx.inFieldList {
		// only resolve column using tables.
		return nr.resolveColumnInTableSources(cn, ctx.tables)
	}
	if ctx.inGroupBy {
		// From tables first, then field list.
		// If ctx.InByItemExpression is true, the item is not an identifier.
		// Otherwise it is an identifier.
		if ctx.inByItemExpression {
			// From table first, then field list.
			if nr.resolveColumnInTableSources(cn, ctx.tables) {
				return true
			}
			found := nr.resolveColumnInResultFields(ctx, cn, ctx.fieldList)
			if nr.Err == nil && found {
				// Check if resolved refer is an aggregate function expr.
				if _, ok := cn.Refer.Expr.(*ast.AggregateFuncExpr); ok {
					nr.Err = ErrIllegalReference.Gen("Reference '%s' not supported (reference to group function)", cn.Name.Name.O)
				}
			}
			return found
		}
		// Resolve from table first, then from select list.
		found := nr.resolveColumnInTableSources(cn, ctx.tables)
		if nr.Err != nil {
			return found
		}
		// We should copy the refer here.
		// Because if the ByItem is an identifier, we should check if it
		// is ambiguous even it is already resolved from table source.
		// If the ByItem is not an identifier, we do not need the second check.
		r := cn.Refer
		if nr.resolveColumnInResultFields(ctx, cn, ctx.fieldList) {
			if nr.Err != nil {
				return true
			}
			if r != nil {
				// It is not ambiguous and already resolved from table source.
				// We should restore its Refer.
				cn.Refer = r
			}
			if _, ok := cn.Refer.Expr.(*ast.AggregateFuncExpr); ok {
				nr.Err = ErrIllegalReference.Gen("Reference '%s' not supported (reference to group function)", cn.Name.Name.O)
			}
			return true
		}
		return found
	}
	if ctx.inHaving {
		// First group by, then field list.
		if nr.resolveColumnInResultFields(ctx, cn, ctx.groupBy) {
			return true
		}
		if ctx.inHavingAgg {
			// If cn is in an aggregate function in having clause, check tablesource first.
			if nr.resolveColumnInTableSources(cn, ctx.tables) {
				return true
			}
		}
		return nr.resolveColumnInResultFields(ctx, cn, ctx.fieldList)
	}
	if ctx.inOrderBy {
		if nr.resolveColumnInResultFields(ctx, cn, ctx.groupBy) {
			return true
		}
		if ctx.inByItemExpression {
			// From table first, then field list.
			if nr.resolveColumnInTableSources(cn, ctx.tables) {
				return true
			}
			return nr.resolveColumnInResultFields(ctx, cn, ctx.fieldList)
		}
		// Field list first, then from table.
		if nr.resolveColumnInResultFields(ctx, cn, ctx.fieldList) {
			return true
		}
		return nr.resolveColumnInTableSources(cn, ctx.tables)
	}
	// In where clause.
	return nr.resolveColumnInTableSources(cn, ctx.tables)
}

// resolveColumnNameInContext looks up and sets ResultField for a column with the ctx.
func (nr *nameResolver) resolveColumnNameInContext(ctx *resolverContext, cn *ast.ColumnNameExpr) bool {
	if ctx.inTableRefs {
		// In TableRefsClause, column reference only in join on condition which is handled before.
		return false
	}
	if ctx.inFieldList {
		// only resolve column using tables.
		return nr.resolveColumnInTableSources(cn, ctx.tables)
	}
	if ctx.inGroupBy {
		// From tables first, then field list.
		// If ctx.InByItemExpression is true, the item is not an identifier.
		// Otherwise it is an identifier.
		if ctx.inByItemExpression {
			// From table first, then field list.
			if nr.resolveColumnInTableSources(cn, ctx.tables) {
				return true
			}
			return nr.resolveColumnInResultFields(cn, ctx.fieldList)
		}
		// Resolve from table first, then from select list.
		found := nr.resolveColumnInTableSources(cn, ctx.tables)
		if nr.Err != nil {
			return found
		}
		// We should copy the refer here.
		// Because if the ByItem is an identifier, we should check if it
		// is ambiguous even it is already resolved from table source.
		// If the ByItem is not an identifier, we do not need the second check.
		r := cn.Refer
		if nr.resolveColumnInResultFields(cn, ctx.fieldList) {
			if nr.Err != nil {
				return true
			}
			if r != nil {
				// It is not ambiguous and already resolved from table source.
				// We should restore its Refer.
				cn.Refer = r
			}
			return true
		}
		return found
	}
	if ctx.inHaving {
		// First group by, then field list.
		if nr.resolveColumnInResultFields(cn, ctx.groupBy) {
			return true
		}
		return nr.resolveColumnInResultFields(cn, ctx.fieldList)
	}
	if ctx.inOrderBy {
		if nr.resolveColumnInResultFields(cn, ctx.groupBy) {
			return true
		}
		if ctx.inByItemExpression {
			// From table first, then field list.
			if nr.resolveColumnInTableSources(cn, ctx.tables) {
				return true
			}
			return nr.resolveColumnInResultFields(cn, ctx.fieldList)
		}
		// Field list first, then from table.
		if nr.resolveColumnInResultFields(cn, ctx.fieldList) {
			return true
		}
		return nr.resolveColumnInTableSources(cn, ctx.tables)
	}
	// In where clause.
	return nr.resolveColumnInTableSources(cn, ctx.tables)
}