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| user_id | child_1 | child_2 | child_3 |
|---|---|---|---|
| 1 | child_a | child_b | null |
| 2 | child_c | null | null |
| 3 | child_x | child_y | child_z |
| user_id | child_1 | child_2 | child_3 |
|---|---|---|---|
| 1 | child_a | child_b | null |
| 2 | child_c | null | null |
| 3 | child_x | child_y | child_z |