GO Classes DPP | GATE CS, DA | DBMS | Relational Division

Answer 1

This query has two separate restrictions:

1. We care only about Chicago Bulls players.

2. Each selected player must have played in every Bulls game.

First build the complete set of Bulls games.

A Bulls game may have the Bulls as the home team or the away team:

$\mathrm{BG}=\pi_{\mathrm{gameID}}\left(\sigma_{\mathrm{homeTeam}=\text{'Chicago Bulls'}\lor\mathrm{awayTeam}=\text{'Chicago Bulls'}}(\mathrm{Game})\right)$

Now obtain the player-game associations:

$\mathrm{PG}=\pi_{\mathrm{playerID},\mathrm{gameID}}(\mathrm{GameStats})$

A tuple in $\mathrm{GameStats}$ indicates that the player actually played in that game.

$\therefore \mathrm{PG}\div\mathrm{BG}$ returns player IDs that occur in $\mathrm{GameStats}$ for every Bulls game.

Finally, restrict the players to the Bulls and return their names:

$\pi_{\mathrm{name}}\left(\sigma_{\mathrm{team}=\text{'Chicago Bulls'}}(\mathrm{Player})\bowtie\mathrm{AllBG}\right)$

Thus A is correct.

B considers only Bulls home games and ignores away games.

C requires only that a player appeared in at least one Bulls game.

D requires the player to have participated in every game in the entire database, not every Bulls game.

Answer : A

Note : Division is only the final step. Before dividing, you must correctly construct both the required game set and the player-game relation.