2023年11月25日

数据库代写 | SQL: Queries, Programming, Triggers

本次美国代写主要为SQL相关的assignment

Conceptual Evaluation Strategy
Semantics of an SQL query defined in terms of the
following conceptual evaluation strategy:
Compute the cross-product of relation-list.
Discard resulting tuples if they fail qualifications.
Delete attributes that are not in target-list.
If DISTINCT is specified, eliminate duplicate rows.
This strategy is probably the least efficient way to
compute a query! An optimizer will find more
efficient strategies to compute the same answers.

atabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke
A Note on Range Variables
Really needed only if the same relation
appears twice in the FROM clause. The
previous query can also be written as:
SELECT S.sname
FROM Sailors S, Reserves R
WHERE S.sid=R.sid AND bid=103
SELECT sname
FROM Sailors, Reserves
WHERE Sailors.sid=Reserves.sid
AND bid=103

Find sailors who’ve reserved at least one boat
Would adding DISTINCT to this query make a
difference?
What is the effect of replacing S.sid by S.sname in
the SELECT clause? Would adding DISTINCT to
this variant of the query make a difference?
SELECT S.sid
FROM Sailors S, Reserves R
WHERE S.sid=R.sidExpressions and Strings
Illustrates use of arithmetic expressions and string
pattern matching: Find triples (of ages of sailors and
two fields defined by expressions) for sailors whose names
begin and end with B and contain at least three characters.
AS and = are two ways to name fields in result.
LIKE is used for string matching. `_’ stands for any
one character and `%’ stands for 0 or more arbitrary
characters.
SELECT S.age, age1=S.age-5, 2*S.age AS age2
FROM Sailors S
WHERE S.sname LIKE ‘B_%B’