Reason of output

Question

table runners

 

| id    | name |

| 1     | John Doe |

| 2     | Jane Doe |

| 3     | Alice Jones |

| 4     | Bobby Louis |

| 5     | Lisa Romero |

 table races

| id    | event                     | winner_id |

| 1     | 100 meter dash   | 2 |

| 2     | 500 meter dash   | 3 |

| 3     | crosscountry         | 2 |

| 4     | triathalon               | NULL |

Output of the query below?

SELECT * FROM runners WHERE id NOT IN (SELECT winner_id FROM races)

 

"Their is no row selected due to 'null' in races table" Please explain the reason of this.

 

Thanks in advance.