mysql: allow some builtin functions to be nullable (#616)

Co-authored-by: Jessica Bellon <10819486+JessTheBell@users.noreply.github.com>
Co-authored-by: Kyle Conroy <kyle@conroy.org>

Author: JessBellon

internal/mysql/functions.go

@@ -18,3 +18,15 @@ func functionReturnType(f string) string {
 		panic(fmt.Sprintf("unknown mysql function type \"%v\"", f))
 	}
 }
+
+// returns true if MySQL function can return null.
+// See: https://dev.mysql.com/doc/refman/8.0/en/function-reference.html
+func functionIsNullable(f string) bool {
+	switch f {
+	case "avg", "sum", "min", "max", "mod",
+		"concat", "left", "find_in_set", "group_concat":
+		return true
+	default:
+		return false
+	}
+}

internal/mysql/parse.go

@@ -450,7 +450,7 @@ func (pGen PackageGenerator) parseSelectAliasExpr(exprs sqlparser.SelectExprs, t
 					Name: returnVal,
 					Type: sqlparser.ColumnType{
 						Type:    funcType,
-						NotNull: true,
+						NotNull: sqlparser.BoolVal(!functionIsNullable(funcName)),
 					},
 				}
 				cols = append(cols, Column{colDfn, ""}) // func returns types don't originate from a table schema