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| id | a | b | c | d | bar |
|---|---|---|---|---|---|
| 16 | a | b | c | d | a; b; c; d |
| 15 | a | b | c | null | a; b; c |
| 14 | a | b | null | d | a; b; d |
| 13 | a | b | null | null | a; b |
| 12 | a | null | c | d | a; c; d |
| 11 | a | null | c | null | a; c |
| 10 | a | null | null | d | a; d |
| 9 | a | null | null | null | a |
| 8 | null | b | c | d | b; c; d |
| 7 | null | b | c | null | b; c |
| 6 | null | b | null | d | b; d |
| 5 | null | b | null | null | b |
| 4 | null | null | c | d | c; d |
| 3 | null | null | c | null | c |
| 2 | null | null | null | d | d |
| 1 | null | null | null | null |