Apagando, renomeando e editando colunas das tabelas

O comando ALTER TABLE serve para editar, adicionar ou apagar uma coluna de um banco já criado e alterar o nome da tabela já existente. Vamos aos exemplos:

Adicionando colunas
ALTER TABLE [nome da tabela] ADD [nome da coluna] [tamanho] [null / not null] AFTER [nome da coluna anterior];

Exemplo:
ALTER TABLE clientes ADD email char(80) not null AFTER fone;

Eliminando colunas
ALTER TABLE [nome da tabela] DROP [nome da coluna];
Exemplo:
ALTER TABLE clientes DROP email;

Alterando colunas
ALTER TABLE [nome da tabela] CHANGE [nome da coluna atual] [novo nome da coluna] [tamanho] [null / not null];

Exemplo:
ALTER TABLE clientes CHANGE fone telefone char(30) not null;

Fonte

[SQL] Joins

No sentido horário, começando pelo meio:

SELECT <select_line> FROM tableA as A INNER JOIN tableB as B ON a.Key = b.Key

SELECT <select_line> FROM tableA as A RIGHT JOIN tableB as B ON a.Key = b.Key

SELECT <select_line> FROM tableA as A RIGHT JOIN tableB as B ON a.Key = b.Key WHERE a.Key IS NULL

SELECT <select_line> FROM tableA as A FULL OUTER JOIN tableB as B ON a.Key = b.Key WHERE a.Key IS NULL OR b.Key IS NULL

SELECT <select_line> FROM tableA as A FULL OUTER JOIN tableB as B ON a.Key = b.Key WHERE b.Key IS NULL

SELECT <select_line> FROM tableA as A LEFT JOIN tableB as B ON a.Key = b.Key WHERE b.Key IS NULL

SELECT <select_line> FROM tableA as A LEFT JOIN tableB as B ON a.Key = b.Key