[SQL ] 請問如何相減再 group 起來?

[fantasywater (prepare myself)]

+-------+-------+--------+---------+
| att1  | att2  | in/our |  time   |
+-------+-------+--------+---------+
|   A   |   B   |   in   |  7:00   |
+-------+-------+--------+---------+
|   C   |   D   |   out  |  7:50   |
+-------+-------+--------+---------+
|   A   |   B   |   out  |  7:20   |
+-------+-------+--------+---------+
|   C   |   D   |   in   |  7:10   |
+-------+-------+--------+---------+

請問如何下 SQL 計算時間差再 group 起來呢..?

結果希望像下面這樣..苦手中..

+-------+-------+-------------------+
| att1  | att2  |     time_diff     |
+-------+-------+-------------------+
|   A   |   B   |        10         |
+-------+-------+-------------------+
|   C   |   D   |        40         |
+-------+-------+-------------------+

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→ elfstart: 第一列算出來是10?還是20