quartz/content/notes/19-advanced-sql-2.md

title	aliases	tags	sr-due	sr-interval	sr-ease
19-advanced-sql-2		info201 lecture	2022-05-26	7	250

CASE

basically a switch statement

• derived valye where calcyulation isnt simple
• standardising values e.g., booleans: t/f, y/n, 0/1
• flipping between row and col orientation (privoting)
  • all students vs all on one students papers

Set operators

relations are sets of tuples ⇒ we can use set operators

combine rows of two table vertically

source table must be compativble

• same set of columns
• same data types
• = same heading

get new table with same headings as inputs

identical rows deleted

union: all rows

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intersect: rows that appear in both

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difference (except, minus): rows in top that arent in bottom

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Aggregation and grouping

group by

• groups rows that have equal values across all the columns in <column-list>
• always used with aggregate function(s) in select clause
• one row in the result for each differect combined value of the grouped columns

• all non computed columns in select clause must normally appear in group y, and vice versa

restricting by groups (having)

• similiar to where, restricts output of group by
• cant include aggregate functions (where can't);

Analytic functions (FYI)

• enchancement of aggregation
• aggregate without reshaping the ouput
• many more functions avaiable
• sliding windows supported
• dont use when simple group by is sufficient

Select

• select
• from
• where
• group by
• having
• order by

order of evaluation

• from
• where
• group by
• having
• select
• order by

Joins

inner join : matching rows only

outer join: may include non-matching rows from the left table, right table, or both tables

also, cross, and semi join

Cross join product

every possible combination of rows from two tables

uses:

• cards
  • suits table
  • ranks table
  • deck = suits cross join ranks
• timetable
  • days table
  • timeslots table
  • timetable = days cross join timeslots

Semi join

output only trows from one table that match rows from the other

inverse is antijoin: ouput only rows from one table that dont match rows from the other

Subqueries

a select expression embedded inside another

• inselect clause to compute value using data from other tables

• in from: inline view

• in where clause with in, all, some , exists

• can refer to data from "outer" expression (correlated subquery)

• tricky to write, so be careful. maybe stepwise

• can rewrite joins as subqueries, but not vice versa

prefer joins to subqueries

how to develop

• identify components opf multi part query
• implement and test the components as separeate select statements
• combine into one query, nesting one within the other.

Inline views

a named subqueriy embedded in the from clause is effectively a temporary view

visible within thescope of current select expression only

saving sub queries (WITH)

• sometimes need to re-use same or very similar subquery severl times in the same query
• with saves named subqueries for later re-use (in the same query)
• visible iwthin scope of entire select statement

Scope in select

• row variable only exist inside the select expression that defines them
• a select expression can only directly refer to items declared in:
  • its own select and from clause
  • select and from clauses of any elclosing expressions
  • any preceedin with expression
• particularly important for correlated subqueries