sqlparser: Remove INTERVAL as general expression

INTERVAL is not a possible general expression, but only used in specific
cases. With the previous change, we already have it all handled for date
functions. The only remaining cases are for + and - where this is
handled.

Those cases are a bit special. You can have an interval on both sides of
a +, but for a - it can only be done on the right hand side.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

go/vt/sqlparser/ast.go

@@ -2154,6 +2154,7 @@ type (
 	// More information available here: https://dev.mysql.com/doc/refman/8.0/en/window-functions-frames.html
 	FramePoint struct {
 		Type FramePointType
+		Unit IntervalTypes
 		Expr Expr
 	}
 
@@ -2194,6 +2195,16 @@ type (
 	FromFirstLastType int8
 )
 
+// DateAddExprType is an enum to get types of DateAddExpr.
+// This can be one of ADDDATE, DATE_ADD or a '+' operator
+// with an interval left or right.
+type DateAddExprType int8
+
+// DateSubExprType is an enum to get types of DateAddExpr.
+// This can be one of SUBDATE, DATE_SUB or a '-' operator
+// with an interval right.
+type DateSubExprType int8
+
 // *********** Expressions
 type (
 	// Expr represents an expression.
@@ -2340,12 +2351,6 @@ type (
 		Expr         Expr
 	}
 
-	// IntervalExpr represents a date-time INTERVAL expression.
-	IntervalExpr struct {
-		Expr Expr
-		Unit IntervalTypes
-	}
-
 	// TimestampFuncExpr represents the function and arguments for TIMESTAMP{ADD,DIFF} functions.
 	TimestampFuncExpr struct {
 		Name  string
@@ -3005,33 +3010,21 @@ type (
 		MatchType  Expr
 	}
 
-	// AdddateExpr represents ADDDATE()
-	// For more information, see https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_adddate
-	AdddateExpr struct {
-		Date Expr
-		Unit IntervalTypes
-		Expr Expr
-	}
-
-	// DateAddExpr represents DATE_ADD()
+	// DateAddExpr represents ADDDATE(), DATE_ADD()
+	// and additions with an interval on the left and right.
 	// For more information, see https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_date-add
 	DateAddExpr struct {
+		Type DateAddExprType
 		Date Expr
 		Unit IntervalTypes
 		Expr Expr
 	}
 
-	// DateSubExpr represents DATE_SUB()
+	// DateSubExpr represents SUBDATE(), DATE_SUB()
+	// and subtractions with an interval on the right.
 	// For more information, see https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_date-sub
 	DateSubExpr struct {
-		Date Expr
-		Unit IntervalTypes
-		Expr Expr
-	}
-
-	// SubdateExpr represents SUBDATE()
-	// For more information, see https://dev.mysql.com/doc/refman/8.0/en/date-and-time-functions.html#function_date-sub
-	SubdateExpr struct {
+		Type DateSubExprType
 		Date Expr
 		Unit IntervalTypes
 		Expr Expr
@@ -3163,7 +3156,6 @@ func (ListArg) iExpr()                             {}
 func (*BinaryExpr) iExpr()                         {}
 func (*UnaryExpr) iExpr()                          {}
 func (*IntroducerExpr) iExpr()                     {}
-func (*IntervalExpr) iExpr()                       {}
 func (*CollateExpr) iExpr()                        {}
 func (*FuncExpr) iExpr()                           {}
 func (*TimestampFuncExpr) iExpr()                  {}
@@ -3210,10 +3202,8 @@ func (*RegexpInstrExpr) iExpr()                    {}
 func (*RegexpLikeExpr) iExpr()                     {}
 func (*RegexpReplaceExpr) iExpr()                  {}
 func (*RegexpSubstrExpr) iExpr()                   {}
-func (*AdddateExpr) iExpr()                        {}
 func (*DateAddExpr) iExpr()                        {}
 func (*DateSubExpr) iExpr()                        {}
-func (*SubdateExpr) iExpr()                        {}
 func (*ArgumentLessWindowExpr) iExpr()             {}
 func (*FirstOrLastValueExpr) iExpr()               {}
 func (*NtileExpr) iExpr()                          {}
@@ -3305,10 +3295,8 @@ func (*RegexpInstrExpr) iCallable()                    {}
 func (*RegexpLikeExpr) iCallable()                     {}
 func (*RegexpReplaceExpr) iCallable()                  {}
 func (*RegexpSubstrExpr) iCallable()                   {}
-func (*AdddateExpr) iCallable()                        {}
 func (*DateAddExpr) iCallable()                        {}
 func (*DateSubExpr) iCallable()                        {}
-func (*SubdateExpr) iCallable()                        {}
 func (*ArgumentLessWindowExpr) iCallable()             {}
 func (*FirstOrLastValueExpr) iCallable()               {}
 func (*NtileExpr) iCallable()                          {}