sqlpass-latam: Outer joins

Scottish Parliament

The data includes all Members of the Scottish Parliament (MSPs) in 1999. Most MSPs belong to a political party. Some parties have a leader who is an MSP. There are two tables:

msp
Name	Party	Constituency
Adam MSP, Brian	SNP	North East Scotland
Aitken MSP, Bill	Con	Glasgow
Alexander MSP, Ms Wendy	Lab	Paisley North
... Total number of records: 129

party
Code	Name	Leader
Con	Conservative	McLetchie MSP, David
Green	Green
Lab	Labour	Dewar MSP, Rt Hon Donald
... Total number of records: 9

Dealing with NULL.

1a	One MSP was kicked out of the Labour party and has no party. Find him. Why we cannot use = You might think that the phrase `party=NULL` would work here. It doesn't. This is because `NULL` "propogates". Any normal expression that includes `NULL` is itself `NULL`, thus the value of the expressions `2+NULL` and `party \|\| NULL` and `NULL=NULL` for example are all `NULL` . theory The `NULL` value does not cause a type error, however it does infect everything it touches with `NULL`-ness. We call this element the `bottom` value for the algebra - but we don't snigger because we are grown-ups. Bottom Type. SELECT name FROM msp WHERE party IS NULL

One MSP was kicked out of the Labour party and has no party. Find him.

Why we cannot use =

You might think that the phrase party=NULL would work here. It doesn't. This is because NULL "propogates". Any normal expression that includes NULL is itself NULL, thus the value of the expressions 2+NULL and party || NULL and NULL=NULL for example are all NULL .

theory

The NULL value does not cause a type error, however it does infect everything it touches with NULL-ness. We call this element the bottom value for the algebra - but we don't snigger because we are grown-ups. Bottom Type.

SELECT name FROM msp WHERE party IS NULL

1b	Obtain a list of all parties and leaders. SELECT name, leader FROM party

Obtain a list of all parties and leaders.

SELECT name, leader FROM party

1c	Give the party and the leader for the parties which have leaders. SELECT name, leader FROM party WHERE leader IS NOT NULL

Give the party and the leader for the parties which have leaders.

SELECT name, leader FROM party
  WHERE leader IS NOT NULL

1d	Obtain a list of all parties which have at least one MSP. SELECT DISTINCT party.name FROM msp, party WHERE party=code

Obtain a list of all parties which have at least one MSP.

SELECT DISTINCT party.name FROM msp, party
  WHERE party=code

2a	Obtain a list of all MSPs by name, give the name of the MSP and the name of the party where available. Be sure that `Canavan MSP, Dennis` is in the list. SELECT msp.name, party.name FROM msp LEFT JOIN party ON party=code ORDER BY msp.name

Obtain a list of all MSPs by name, give the name of the MSP and the name of the party where available. Be sure that Canavan MSP, Dennis is in the list.

SELECT msp.name, party.name
  FROM msp LEFT JOIN party ON party=code
  ORDER BY msp.name

2b	Obtain a list of parties together with the number of MSPs. SELECT party.name, COUNT(msp.name) FROM msp, party WHERE msp.party=party.code GROUP BY party.name

Obtain a list of parties together with the number of MSPs.

SELECT party.name, COUNT(msp.name)
  FROM msp, party
  WHERE msp.party=party.code
  GROUP BY party.name

2c	A list of parties with the number of MSPs; include parties with no MSPs. SELECT party.name, COUNT(msp.name) FROM party LEFT JOIN msp ON party.code=msp.party GROUP BY party.name

A list of parties with the number of MSPs; include parties with no MSPs.

SELECT party.name, COUNT(msp.name)
  FROM party LEFT JOIN msp ON party.code=msp.party
  GROUP BY party.name

A Gentle Introduction to

SQL

Scottish Parliament

Dealing with `NULL`.

A Gentle Introduction to

SQL

Scottish Parliament

Dealing with NULL.

Dealing with `NULL`.