feat: add substring and substr for field String (#1194)

* feat: add substring and substr for field String

* feat: add comment for substring

Author: qqxhb

do_test.go

@@ -141,6 +141,24 @@ func TestDO_methods(t *testing.T) {
 			ExpectedVars: []interface{}{uint(10)},
 			Result:       "WHERE `id` = ?",
 		},
+		{
+			Expr:   u.Where(u.Name.Substring(1)),
+			Result: "WHERE SUBSTRING(`name`,1)",
+		},
+		{
+			Expr:         u.Where(u.Name.Substring(1, 6), u.ID.Eq(10)),
+			ExpectedVars: []interface{}{uint(10)},
+			Result:       "WHERE SUBSTRING(`name`,1,6) AND `id` = ?",
+		},
+		{
+			Expr:         u.Where(u.Name.Substr(1), u.ID.Eq(10)),
+			ExpectedVars: []interface{}{uint(10)},
+			Result:       "WHERE SUBSTR(`name`,1) AND `id` = ?",
+		},
+		{
+			Expr:   u.Where(u.Name.Substr(1, 6)),
+			Result: "WHERE SUBSTR(`name`,1,6)",
+		},
 		{
 			Expr:         u.Where(u.Name.Eq("tom"), u.Age.Gt(18)),
 			ExpectedVars: []interface{}{"tom", 18},

field/string.go

@@ -147,6 +147,41 @@ func (field String) SubstringIndex(delim string, count int) String {
 	}}}
 }
 
+// Substring https://dev.mysql.com/doc/refman/8.4/en/string-functions.html#function_substring
+func (field String) Substring(params ...int) String {
+	if len(params) == 0 {
+		return field
+	}
+	if len(params) == 1 {
+		return String{expr{e: clause.Expr{
+			SQL:  fmt.Sprintf("SUBSTRING(?,%d)", params[0]),
+			Vars: []interface{}{field.RawExpr()},
+		}}}
+	}
+	return String{expr{e: clause.Expr{
+		SQL:  fmt.Sprintf("SUBSTRING(?,%d,%d)", params[0], params[1]),
+		Vars: []interface{}{field.RawExpr()},
+	}}}
+}
+
+// Substr SUBSTR is a synonym for SUBSTRING 
+// https://dev.mysql.com/doc/refman/8.4/en/string-functions.html#function_substring
+func (field String) Substr(params ...int) String {
+	if len(params) == 0 {
+		return field
+	}
+	if len(params) == 1 {
+		return String{expr{e: clause.Expr{
+			SQL:  fmt.Sprintf("SUBSTR(?,%d)", params[0]),
+			Vars: []interface{}{field.RawExpr()},
+		}}}
+	}
+	return String{expr{e: clause.Expr{
+		SQL:  fmt.Sprintf("SUBSTR(?,%d,%d)", params[0], params[1]),
+		Vars: []interface{}{field.RawExpr()},
+	}}}
+}
+
 func (field String) toSlice(values []string) []interface{} {
 	slice := make([]interface{}, len(values))
 	for i, v := range values {