Improved creating multiple table joins. Closes #6. Closes #7.

Creating a single table join remains the same, there is no breaking change to the API. However, creating two or more table joins is now more intuitive. The return value from 'table1AndTable2 = sqlSelect.Tables.Joins.Add(table1, Type.Inner, table2)' can then be used in subsequent table join calls i.e. 'sqlSelect.Tables.Joins.Add(table1AndTable2 , Type.Inner, table3)'. This indicates that the table1AndTable2 join created in the first step is then joined to table3.

Also changed the SQLSelectTableJoinCondition to accept SQLExpression objects rather than the left and right table field names. This was required because now that the left table for a join can be either a table or a joined table the left field table alias may be unknown if it is an table join. In which case the table alias must come from an SQLFieldExpression (inherits SQLExpression) which will indicate the table alias. The way it was working where it would obtain the table name from the join was incorrect. So also closing issue #7.

SQL/Select/SQLSelectTableBase.vb

@@ -16,7 +16,7 @@ Namespace SQL
         Protected MustOverride ReadOnly Property Source(ByVal eConnectionType As Database.ConnectionType) As String
         Friend MustOverride Function GetPrefix() As String
 
-        Private pstrAlias As String
+        Private pstrAlias As String = String.Empty
 
         Public Sub New()
 

SQL/Select/SQLSelectTableJoin.vb

@@ -12,6 +12,7 @@ Option Explicit On
 Namespace SQL
 
     Public Class SQLSelectTableJoin
+        Inherits SQLSelectTableBase
 
         Public Enum Type
             Inner
@@ -94,30 +95,17 @@ Namespace SQL
             End Set
         End Property
 
-        Friend ReadOnly Property SQL(ByVal eConnectionType As Database.ConnectionType) As String
-            Get
+        Friend Overrides Function GetPrefix() As String
 
-                Dim strSQL As String = String.Empty
-                Dim strJoin As String = String.Empty
-
-                Select Case Me.TheType
-                    Case SQLSelectTableJoin.Type.Inner
-                        strJoin = "INNER JOIN"
-                    Case SQLSelectTableJoin.Type.FullOuter
-                        strJoin = "FULL OUTER JOIN"
-                    Case SQLSelectTableJoin.Type.LeftOuter
-                        strJoin = "LEFT OUTER JOIN"
-                    Case SQLSelectTableJoin.Type.RightOuter
-                        strJoin = "RIGHT OUTER JOIN"
-                End Select
-
-                If Index() = 0 Then
-                    strSQL = pobjLeftTable.SQL(eConnectionType)
-                Else
-                    strSQL = pobjParent(Index() - 1).SQL(eConnectionType)
-                End If
+            Return MyBase.Alias
+
+        End Function
 
-                strSQL &= " " & strJoin & " " & pobjRightTable.SQL(eConnectionType)
+        Protected Overrides ReadOnly Property Source(ByVal eConnectionType As Database.ConnectionType) As String
+            Get
+
+                Dim strSQL As String = _
+                    pobjLeftTable.SQL(eConnectionType) & " " & GetJoinString(Me.TheType) & " " & pobjRightTable.SQL(eConnectionType)
 
                 If Not pobjConditions Is Nothing AndAlso Not pobjConditions.IsEmpty Then
                     strSQL &= " ON " & pobjConditions.SQL(eConnectionType)
@@ -129,13 +117,20 @@ Namespace SQL
             End Get
         End Property
 
-        Private Function Index() As Integer
-
-            For intIndex As Integer = 0 To pobjParent.Count - 1
-                If pobjParent(intIndex) Is Me Then
-                    Return intIndex
-                End If
-            Next
+        Private Shared Function GetJoinString(eJoinType As Type) As String
+
+            Select Case eJoinType
+                Case SQLSelectTableJoin.Type.Inner
+                    Return "INNER JOIN"
+                Case SQLSelectTableJoin.Type.FullOuter
+                    Return "FULL OUTER JOIN"
+                Case SQLSelectTableJoin.Type.LeftOuter
+                    Return "LEFT OUTER JOIN"
+                Case SQLSelectTableJoin.Type.RightOuter
+                    Return "RIGHT OUTER JOIN"
+                Case Else
+                    Throw New NotImplementedException(eJoinType.ToString)
+            End Select
 
         End Function
 

SQL/Select/SQLSelectTableJoinCondition.vb

@@ -13,9 +13,9 @@ Namespace SQL
 
     Public Class SQLSelectTableJoinCondition
 
-        Private pstrLeftTableFieldName As String
+        Private pobjLeftExpression As SQLExpression
         Private peCompare As ComparisonOperator
-        Private pstrRightTableFieldName As String
+        Private pobjRightExpression As SQLExpression
         Private pobjParent As SQLSelectTableJoinConditions
 
         Friend Sub New(ByVal objParent As SQLSelectTableJoinConditions)
@@ -24,16 +24,16 @@ Namespace SQL
 
         End Sub
 
-        Public Property RightTableFieldName() As String
+        Public Property RightExpression() As SQLExpression
             Get
 
-                Return pstrRightTableFieldName
+                Return pobjRightExpression
 
             End Get
 
-            Set(ByVal Value As String)
+            Set(ByVal Value As SQLExpression)
 
-                pstrRightTableFieldName = Value
+                pobjRightExpression = Value
 
             End Set
         End Property
@@ -52,38 +52,27 @@ Namespace SQL
             End Set
         End Property
 
-        Public Property LeftTableFieldName() As String
+        Public Property LeftExpression() As SQLExpression
             Get
 
-                Return pstrLeftTableFieldName
+                Return pobjLeftExpression
 
             End Get
 
-            Set(ByVal Value As String)
+            Set(ByVal Value As SQLExpression)
 
-                pstrLeftTableFieldName = Value
+                pobjLeftExpression = Value
 
             End Set
         End Property
 
         Friend ReadOnly Property SQL(ByVal eConnectionType As Database.ConnectionType) As String
             Get
 
-                Dim strSQL As String
-
-                If LeftTableFieldName.Trim = String.Empty Then
-                    Throw New Exceptions.DatabaseObjectsException("LeftTableFieldName has not been specified.")
-                End If
-
-                If RightTableFieldName.Trim = String.Empty Then
-                    Throw New Exceptions.DatabaseObjectsException("RightTableFieldName has not been specified.")
-                End If
-
-                strSQL = _
-                    SQLFieldNameAndTablePrefix(pobjParent.Parent.LeftTable, LeftTableFieldName, eConnectionType) & " " & _
-                    SQLConvertCompare(Compare) & " " & SQLFieldNameAndTablePrefix(pobjParent.Parent.RightTable, RightTableFieldName, eConnectionType)
-
-                Return strSQL
+                Return _
+                    pobjLeftExpression.SQL(eConnectionType) & " " & _
+                    SQLConvertCompare(Compare) & " " & _
+                    pobjRightExpression.SQL(eConnectionType)
 
             End Get
         End Property

SQL/Select/SQLSelectTableJoinConditions.vb

@@ -44,25 +44,61 @@ Namespace SQL
             ByVal eCompare As ComparisonOperator, _
             ByVal strRightTableFieldName As String) As SQLSelectTableJoinCondition
 
-            'Add the AND operator if an operator hasn't been called after the previous Add call
-            If pobjLogicalOperators.Count() < pobjConditions.Count() Then
-                Me.AddLogicalOperator(LogicalOperator.And)
+            'The Add function is here basically for backward compatibility when the conditions could only accept field names and the left and right tables from the parent join were used as table aliases. 
+            'Now that that SQLSelectTableBase is being used (which can represent a table or joined tables) we need to check that parent left and right tables are only SQLSelectTable objects.
+            If Not TypeOf pobjParent.LeftTable Is SQLSelectTable Then
+                Throw New ArgumentException("The left table is expected to be an SQLSelectTable so that the left table alias can be utilised for the strLeftTableFieldName argument. Use the Add(SQLExpression, ComparisonOperator, SQLExpression) overload instead.")
+            ElseIf Not TypeOf pobjParent.RightTable Is SQLSelectTable Then
+                Throw New ArgumentException("The right table is expected to be an SQLSelectTable so that the right table alias can be utilised for the strRightTableFieldName argument. Use the Add(SQLExpression, ComparisonOperator, SQLExpression) overload instead.")
             End If
 
-            If eCompare = ComparisonOperator.Like Or eCompare = ComparisonOperator.NotLike Then
-                Throw New Exceptions.DatabaseObjectsException("LIKE operator is not supported for table joins.")
-            End If
+            EnsureComparisonOperatorValid(eCompare)
+            AddLogicalOperatorIfRequired()
+
+            Add = New SQLSelectTableJoinCondition(Me)
+            Add.LeftExpression = New SQLFieldExpression(DirectCast(pobjParent.LeftTable, SQLSelectTable), strLeftTableFieldName)
+            Add.Compare = eCompare
+            Add.RightExpression = New SQLFieldExpression(DirectCast(pobjParent.RightTable, SQLSelectTable), strRightTableFieldName)
+
+            pobjConditions.Add(Add)
+
+        End Function
+
+        Public Function Add( _
+            ByVal objLeftExpression As SQLExpression, _
+            ByVal eCompare As ComparisonOperator, _
+            ByVal objRightExpression As SQLExpression) As SQLSelectTableJoinCondition
+
+            EnsureComparisonOperatorValid(eCompare)
+            AddLogicalOperatorIfRequired()
 
             Add = New SQLSelectTableJoinCondition(Me)
 
-            Add.LeftTableFieldName = strLeftTableFieldName
+            Add.LeftExpression = objLeftExpression
             Add.Compare = eCompare
-            Add.RightTableFieldName = strRightTableFieldName
+            Add.RightExpression = objRightExpression
 
             pobjConditions.Add(Add)
 
         End Function
 
+        Private Sub AddLogicalOperatorIfRequired()
+
+            'Add the AND operator if an operator hasn't been called after the previous Add call
+            If pobjLogicalOperators.Count() < pobjConditions.Count() Then
+                Me.AddLogicalOperator(LogicalOperator.And)
+            End If
+
+        End Sub
+
+        Private Sub EnsureComparisonOperatorValid(eCompare As ComparisonOperator)
+
+            If eCompare = ComparisonOperator.Like Or eCompare = ComparisonOperator.NotLike Then
+                Throw New Exceptions.DatabaseObjectsException("LIKE operator is not supported for table joins.")
+            End If
+
+        End Sub
+
         Public Sub AddLogicalOperator( _
             ByVal eLogicalOperator As LogicalOperator)
 

SQL/Select/SQLSelectTableJoins.vb

@@ -33,9 +33,9 @@ Namespace SQL
         End Function
 
         Public Function Add( _
-            ByVal objLeftTable As SQLSelectTable, _
+            ByVal objLeftTable As SQLSelectTableBase, _
             ByVal eJoin As SQLSelectTableJoin.Type, _
-            ByVal objRightTable As SQLSelectTable) As SQLSelectTableJoin
+            ByVal objRightTable As SQLSelectTableBase) As SQLSelectTableJoin
 
             Dim objJoin As SQLSelectTableJoin = New SQLSelectTableJoin(Me)