DBMS Week 2 pt.1

    Cards (15)

    • What does relational algebra tell a database exactly?
      How to manipulate data
    • What is the symbol for the selection operation?
      σ\sigma
    • What does the projection operation retrieve?
      Specific columns
    • Match the relational algebra operation with its symbol and description.
      1️⃣ Selection: σ, Filters rows based on a condition
      2️⃣ Projection: π, Retrieves specific columns
      3️⃣ Union: ∪, Combines rows from two relations
      4️⃣ Set Difference: -, Finds rows present in the first relation but not in the second
      5️⃣ Intersection: ∩, Identifies common rows in two relations
      6️⃣ Cartesian Product: X, Connects every row from one relation to every row in another
      7️⃣ Join: ⋈, Combines tuples from two relations based on a condition
    • Match the relational algebra operation with its example.
      Selection ↔️ σ\sigmaDept="Sales" (Employee)
      Projection ↔️ π\piName,Salary (Employee)
      Union ↔️ Student \cup Instructor
    • What type of subset does the projection operation create?
      Vertical subset
    • Write the relational algebra expression to find the names of employees in the Sales department with salaries over $50,000.
      π\piName (σ\sigmaDepartment = "Sales" AND Salary > 50000 (Employee))
    • Arrange the following relational algebra operations in the correct sequence to find the names of employees in the Sales department with salaries over $50,000.
      1️⃣ Selection: σ\sigmaDepartment = "Sales" (Employee)
      2️⃣ Selection: σ\sigmaSalary > 50000
      3️⃣ Projection: π\piName
    • What does it mean for two relations to be union-compatible?
      Matching attributes
    • What does the intersection operation identify in two relations?
      Common tuples
    • Steps to use the rename operation
      1️⃣ Rename the relation
      2️⃣ Rename the attributes
      3️⃣ Rename both the relation and attributes
    • What does a natural join combine relations on?
      Common attributes
    • Match the aggregate function with its purpose in relational algebra:
      COUNT ↔️ Counts rows
      SUM ↔️ Sums numeric values
      AVG ↔️ Calculates average
      MIN ↔️ Finds minimum value
    • Which relational operator is allowed in theta join conditions?
      Equality ||| Greater than ||| Less than
    • What do aggregate functions return in relational algebra?
      Single value
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