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data engineering > no sql > Flashcards

no sql Flashcards

1
Q

what are the 4 types of data

A

structured and unstructured
dynamic and static

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2
Q

dynamic

A

changing frequently

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3
Q

static

A

never changes

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4
Q

strcutured

A

formal predefined
easy to store and process

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5
Q

unstructured

A

e.g. audio, image, music
usually still has internal structural properties

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6
Q

sharding data

A

splitting the data to allow concurrent/parallel access using multiple machines
can simultaneously access each shard

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7
Q

in which two ways can we scale databases

A

vertically and horizontally

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8
Q

vertical scaling

A

upgrading hardware e.g. increasing memory

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9
Q

what is the limitaiton of vertical scaling

A

limited by the amount of cpu ram disk etc that can be configured on a single machine

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10
Q

horizontal scaling

A

adding more machines which requires shading and replication so you can work with them simultaneously

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11
Q

what is the limitation of horizontal scaling

A

read-to-write ratio and communication overhead

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12
Q

in which three ways can we benefit form parallelisation

A

maximise the fraction of the program that can be parallelised
balance the work load and parallel process
minimise the time spent on communication

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13
Q

how does the two phase commit protocol work

A

the coordinator requests cote for commit and the participants either approve or reject
if all participants accept then everything gets committed at the same time

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14
Q

what is the issue with two phase commit

A

hard to find a time where all servers are ready to commit

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15
Q

what is the CAP theorem

A

any distributed database with shared data can have at most 2/3
usually sacrificing consistency

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16
Q

what are the three components of cap theorem

A

consistency; every node always sees the same data at the same time
availability; the system continues operating even if nodes crash or software or hardware is down
partition tolerance; the system works well when distributed

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17
Q

what are the BASE properties

A

basically available; the system guaranteed availability
soft state; system state may change over time
eventual consistency; will eventually become consistent

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18
Q

what does it mean for a db to be eventually consistent

A

if all replicas will gradually become consistent in the absence of updates

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19
Q

what makes no sql no sql

A

no strict schema requirements
no strict adherence to acid properties
consistency is traded in favour of availability

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20
Q

document database/store

A

loosely structured set of key value pairs in documents encapsulate and encode data in some standard formats/encodings
treated as a whole
query languages can help retrieve documents based on their contents
addressed in the db via the unique key

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21
Q

in mongo what is the primary key

A

key; “_id”

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22
Q

sorted ordered column-oriented stores

A

columns are grouped in column families which data is stored in rather than tables
each unit of data is a set of key value pairs identified by row-key

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23
Q

graph db

A

everything is stored as an edge node or attribute
each node and edge can have any number attributes and can be labelled which narrows searches

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24
Q

what do document db use instead of an fk

A

embedded documents and referencing

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25
Q

what can be a value in document db

A

any data type

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26
Q

references

A

including links from one document in another which normalises the db

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27
Q

what are some benefits of using referencing

A

can represent more complex many-to-many relationships
good for large hierarchical datasets

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28
Q

what is a negative of using referencing

A

requires follow up queries to find all the data you need

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29
Q

embedded data

A

having a doc inside another via an array

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30
Q

embedded data positive

A

can get all the data in one call using less queries

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31
Q

negative of embedded data

A

the db isn’t normalised an not all values are atomic

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32
Q

data model

A

displays a set of tables and the relationship between them providing a blueprint so you can identify which data is important and what should be maintained

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33
Q

which two parts of the CAP approach does mongodb focus on

A

consistency and partition tolerance

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34
Q

what are the different parts of the mongodb structure and how do they relate to eachother

A

an instance has 0/more databases
a database has 0/more collections
a collection has 0/more documents
a document has 1/more fields/attributes

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35
Q

what is mongosh

A

an interactive shell that is a fully functional javascrips interpreter

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36
Q

in mongo what happens when you USE a db but it doesnt exist

A

mongo creates it

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37
Q

db.dropDatabase

A

deleted the db

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38
Q

show dbs

A

shows your databases

39
Q

what does db.collection.findOne({“title”: /c/}) do

A

finds all the attributes with c in the name (regular expressions)

40
Q

mongod

A

db instance

41
Q

CRUD

A

create
read
update
delete

42
Q

db.collection.insertOne()

A

inserts a single document into the collection and if the collection doesnt exist then mongo creates it

43
Q

what does the backtick do``

A

evaluates the contents within it

44
Q

db.collection.find

A

prints all docs in the collection

45
Q

db.collection.find(<query>)</query>

A

prints all docs in the collection that match the query

46
Q

db.collection.find(<query>).count</query>

A

prints the number of docs in the collection that match the query

47
Q

db.collection.findOne()

A

prints the first doc that matches the query

48
Q

what happens if the projection and query are both null (db.collection.findOne(<query>,<projection>)</projection></query>

A

prints out all the fields in the first document

49
Q

db.collection.updateOne((<query,<update>,<options>)</options></update>

A

updates the command matching the query

50
Q

db.collection.updateMany((<query,<update>,<options>)</options></update>

A

updates all documents matching the query

51
Q

in which two ways do we enforce constraints in mongodb

A

validators and indexes

52
Q

validator

A

ensure fields meet specific criteria
e.g. required, not null, datatype

53
Q

indexes

A

can enforce uniqueness and speeds up searching by organising data and provide multiplicity constraints
e.g. unique

54
Q

index 1

A

ascending

55
Q

index -1

A

descending

56
Q

how do we use indexes to create m:n relationships

A

creating a compound index for the join table and make them unique

57
Q

validator + index

A

exactly 1

58
Q

using index means

A

at most one

59
Q

using validator means

A

at least one

60
Q

what is the main difference between validators and indexes when being created

A

validators are created while creating the collection whilst indexes can be created after

61
Q

what is the key symbol for a query

A

$

62
Q

$gt

A

greater than

63
Q

$eq

A

equal

64
Q

$and

A

AND

65
Q

$or

A

OR

66
Q

$regex

A

regular expression

67
Q

what does {} signify in a findOne()

A

the condition

68
Q

how can we ensure total participation using a validator

A

required: [x,y]

69
Q

,

A

implicit AND

70
Q

what does a regex start and end with

A

/ and $/

71
Q

^ meaning in regex

A

start of the string

72
Q

. meaning in regex

A

any character

73
Q

{5,} meaning in regex

A

at least 5

74
Q

i meaning in regex

A

case insensitive

75
Q

?! meaning in regex

A

negative lookahead; checks if the pattern does not follow the current position

76
Q

?= meaning in regex

A

positive lookahead; checks if the pattern follows the current position

77
Q
  • meaning in regex
A

order of string not important

78
Q

what is the cursor in mongodb

A

a pointer that keeps the value returned by find()

79
Q

how do you retrieve documents as a constant value from the cursor

A

using loops

80
Q

.sort({item: 1/-1})

A

sorts the returned items in ascending or descending order

81
Q

how do we show which attributes we want to be included in the result of find()

A

at the v end of the query
name: 1

82
Q

how do we omit the _id from our find() result

A

_id: 0
can only be used with _id as its automatically given

83
Q

how do we use IN in mongodb

A

type: {$in:[‘food’]}

84
Q

how do we search in an embedded document

A

need to fill all subdocuments fields and in the same order when making your query

85
Q

how do we get the first value in an array when writing a query

A

e.g. ‘ratings.0’

86
Q

what does $elemMatch do

A

returns if at least one array element satisfies the entire condition

87
Q

why do we need to use $elemMatch

A

if we dont and we have multiple conditions then mongo just does an implicit or

88
Q

what is the mongo equivalent of group by having

A

$group $match

89
Q

aggregation pipeline

A

documents go through a pipeline of operations until aggregated

90
Q

how do you use count to find the number of documents in the collection

A

$count : “name”

91
Q

how do you count a specific attribute in a document in a collection

A

$match before counting

92
Q

db.createView(viewName, collection, pipeline, options)

A

temporarily returns a collection that will not be kept in the database

93
Q

$lookup

A

creates a cross product of two collections based on a common field

94
Q

$unwind

A

deconstructs/ flattens an array into individual docs

data engineering (7 decks)

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