Table: ta_a_portugal_0607
Columns: round
date_s
clubs_from_the_previous_round
clubs_involved
fixtures

Question: When 4 clubs are involved, what is the average number of fixtures?
SQL: SELECT AVG("fixtures") FROM ta_a_portugal_0607 WHERE "clubs_involved" = 4

Question: In the sixth round, what is the sum of fixtures when there were more than 15 clubs involved?
SQL: SELECT SUM("fixtures") FROM ta_a_portugal_0607 WHERE "round" = 'sixth round' AND "clubs_involved" > 15