Database Management Essentials: Module 4 Basic Query Formulation with SQL

Database Management
Essentials
Basic Query Formulation with SQL
Summarized by
github.com/dgkim5360
dgkim5360.tistory.com

SQL Overview
Structured Query Language

A computer language 
containing statements of database 
definition / control / manipulation
Context

• Stand-alone: Workbench

• Embedded: Django App

Weakness

표준화가 엉망이다: Higher switching cost
Major SQL Statements
Statement Statement Type
CREATE TABLE Definition, Control
CREATE VIEW Definition
CREATE TYPE Definition
SELECT Manipulation
INSERT Manipulation
UPDATE Manipulation
DELETE Manipulation
COMMIT Manipulation
ROLLBACK Manipulation
CREATE TRIGGER Control, Manipulation
GRANT Control
REVOKE Control

SELECT Statement
Introduction
SELECT <list-of-column-expression> 
FROM <list-of-tables-and-join-expression> 
WHERE <list-of-logical-expression-for-rows> 
ORDER BY <list-of-sorting-specifications> 
;
Additional Keywords

•AND, OR
•DISTINCT: Remove duplicated rows
•AS: Rename a column
•LIKE: Inexact string matching 
DATE type에는 사용하지 말자 
Treat DATE as numeric, not text
•BETWEEN AND
•IS NULL: Testing a lack of value
Portability Issue

e.g. Diﬀerent date functions in

• Oracle: to_char, to_string
• MySQL: date_format

JOIN Operator
• Builds a new table by combining rows 
from two tables that match on a join condition

• Unqualified Columns

• Qualified Column? 특정 테이블에 속한 column

• e.g. PK, FK

• EQUI-JOIN (Equality Join)

• 같은 unqualified column을 join column으로 맞춘다

• 보통 PK - FK 간의 연결을 주로 볼 수 있음

Using JOIN in SELECT
Cross-Product Style

SELECT
OfferNo,
CourseNo,
FacFirstName,
FacLastName
FROM Offering, Faculty
WHERE OffTerm = 'FALL'
AND OffYear = 2016
AND FacRank = 'ASST'
AND CourseNo LIKE 'IS%'
AND Faculty.FacNo = Offering.FacNo
;
Join Operator Style

SELECT
OfferNo,
CourseNo,
FacFirstName,
FacLastName
FROM Offering
INNER JOIN Faculty
ON Faculty.FacNo = Offering.FacNo
WHERE OffTerm = 'FALL'
AND OffYear = 2016
AND FacRank = 'ASST'
AND CourseNo LIKE 'IS%'
• N-1 JOIN conditions for N-table JOIN

• 이를 위반하게 되면 심각한 오류가 발생한다.

• 정확하지 않은 결과

• 높은 수준의 자원 소비

• Name Qualiﬁcation 필요

GROUP BY Clause
Row Summaries

SELECT StdMajor, AVG(StdGPA) as AvgGPA
FROM Student
WHERE StdClass IN ('JR', 'SR')
GROUP BY StdMajor
HAVING AVG(StdGPA) > 3.1
;
Query Clause Evaluation Order

• Row Operations

•FROM
•WHERE
• Group Operations (오직 한 번만 일어남)

•GROUP BY
•HAVING
• Final Operations for Results

•ORDER BY
•SELECT
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Database Management Essentials: Module 4 Basic Query Formulation with SQL

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