Skip to content

DBMS

DBMS#

Table of Contents

Basics#

Transaction#

  • represents any changes in database
  • a unit of work performed in DBMS against a database
  • independent of other transactions
  • 2 Purposes:
    • provide reliability, can be recovered on failure
    • isolation from other programs accessing DB concurrently to avoid errors

ACID#

A set of properties of relational database transaction.

  • Atomicity: All or nothing.
  • Consistency: One state to another valid state (according to rulesets, constraints defined).
  • Isolation: Concurrency control. Execution of multiple concurrent transaction should have the same result as executed sequentially.
  • Durability: Changes should persist (permanent is a big word :D).

Rollback#

Cascade#

Keys#

Primary Key#

  • A primary is a single column value used to identify a database record uniquely.
  • It has following attributes:
    • A primary key cannot be NULL
    • A primary key value must be unique
    • The primary key values cannot be changed
    • The primary key must be given a value when a new record is inserted.

Q. Which is better? Auto-incremental number vs UUID as primary key?

Auto-incremental number

  • pros
    • good for internal use
    • small in size (say 8 bytes)
    • fast
    • easy to sort by
  • cons
    • could reveal info/confidentiality
      • a user can guess ID of other
    • in long run, (i.e. very huge data, 10s of billions) integer takes more space than string

UUID

  • pros
    • good for external use
    • unique not only across table, but accross company, or even world
    • does not reveal any info implicitly
      • hence more secure system
    • easier to merge data in sub-database/sub-table
  • cons
    • relatively more space (say 16 bytes)
    • performace disaster for very large tables (say more than 200K)
    • aer very random, so using them as unique/primary key (basically indexing) is inefficient on very large tables
    • cannot sort
    • could be adhoc (if we still need a number based unique key as well, say rollno, empid etc)
      • so space for index for uuid as well as for sequence
      • having both is waste

Ref: - https://stackoverflow.com/questions/33274291/uuid-or-sequence-for-primary-key/33274393#33274393 - https://dba.stackexchange.com/questions/115766/should-i-use-uuid-as-well-as-id/119129 - http://compwron.github.io/2016/06/15/uuids-ids-and-primary-keys.html

UUID4#
  • UUID version 4
  • 128 bits
  • 122 for data/randomization (i.e. could have 122 random bits)
  • 6 bits for identification of UUID version
How to generate a custom UUID?#
  • machine id (MAC?) + timestamp (nano) + process id + thread id + counter
  • may keep a lookup cache of 1 second

Composite Key#

  • A composite key is a primary key composed of multiple columns used to identify a record uniquely

Foreign Key#

  • A key which is referring to a primary key of another table

Candidate Key#

  • Candidate for the primary key. Means whichever combination of columns can uniquely identify a record are the candidate keys.

Super Key#

  • Superset of all the candidate key. Means a set which contains all the columns from all the candidate keys.

Dependencies#

Functional Dependency#

Partial Dependency#

Transitive Dependency#

Normalization#

  • A technique of organizing data in DB
  • A systematic approach to decomposing tables to eliminate data redundancy & undesired characteristics like:
    • Insertion Anamolies
    • Update Anamolies
    • Deletion Anamolies
  • Multi step process
  • Normalization is used for mainly two purposes:
    • Eliminating redundant(useless) data.
    • Ensuring data dependencies make sense i.e data is logically stored.

Note: In most practical applications, normalization achieves its best in 3rd Normal Form.

Anamolies#

  • Insertion Anamoly
    • Suppose for a new admission, we have a Student id, name, and address of a student but if the student has not opted for any subjects yet then we have to insert NULL there, leading to Insertion Anamoly.
  • Update Anamoly
    • To update the address of a student who occurs twice or more than twice in a table, we will have to update S_Address column in all the rows, else data will become inconsistent.
  • Deletion Anamoly
    • If a student has only one subject and temporarily he drops it. When we delete that row, entire student record will be deleted along with it.

Types#

0 NF: Un Normalized Form#
  • Multi Valued Cells like Unit Code
1 NF (E. F. Codd - 1971)#
  • First normal form enforces these criteria:
    • Eliminate repeating groups in individual tables.
    • Create a separate table for each set of related data.
    • Identify each set of related data with a primary/composite key
  • Rules
    • Atomic (i.e. indivisible) / single values in each cell. Means no set of values in a single cell.
    • Values stored in a column should be of the same domain
      • i.e. No repeating group/column/attribute. like Telephone 1, Telephone 2
    • All the columns in a table should have unique names.
    • No repeating rows/records. In other way:
      • For a set of values, create multiple rows/records for each individual values in the set.
      • Or, Identify each row/records by a primary key/composite primary key
  • Disadvantage:
    • Using the First Normal Form, data redundancy increases, as there will be many columns with same data in multiple rows but each row as a whole will be unique.

Table: 1-NF

Table: Still in 1-NF

  • 2 NF
    • Be in 1 NF
    • There must not be a Partial Dependency Table: 2-NF
  • 3 NF
  • BCNF

Index#

  • are used to quickly locate data without searching every row in the table

Why#

Type#

How#

Implementation#

Doubts/FAQs#

  • why not use hash table instead b-tree?
    • https://stackoverflow.com/questions/7306316/b-tree-vs-hash-table
    • https://www.quora.com/A-B-tree-index-is-better-than-a-hash-index-Why
    • https://dev.mysql.com/doc/refman/8.0/en/index-btree-hash.html
    • read sqlite code/doc as well
    • https://hakibenita.com/postgresql-hash-index
  • how a really huge b-tree fits in RAM?
    • https://stackoverflow.com/questions/764221/larger-than-memory-data-structures-and-how-they-are-typically-handled
    • what is paging?
  • primary key uses indexing by default?
    • Yes
  • when not to use DB index?
    • when table is small
    • on columns on which majority of the values are null
    • on columns which are frequently manipulated
    • on columns which returns a high percentage of rows if use filter condition (like WHERE clause) on those
    • indexing could perform slow on tables which get frequent bulk updates

SQL#

Stored Procedure & Function#

Are schema level subprograms/program unit/commonly used codes, stored in database.

Procedure Function
Stored Procedures can call functions. Functions cannot call stored Procedures.
Can have select statements as well as DML statements Cannot use DML statements
Can use both table variables as well as temporary table in it. Cannot use temp tables
Procedures cannot be utilized in a select statement Function can be embedded in a select statement.
Procedure can return multiple OUT values(max. 1024) Function returns 1 value only however it can be collection datatype

How to optimize a query? a SP? a Func?#

Cursor? What? Why? How? When? Type?#

Trigger#

Jobs?#

Dynamic SQL?#

Generally Asked#

  • second highest salary, query emp, dept, city
  • import excel to table
  • Employees with third highest salary @ address Pune? Tables: Employees, Address, Salary?
  • delete vs drop vs truncate