SELECT * , ALIAS
Autor: Jeff-DBA | Publicado: 2011-08-25T19:00:00-07:00

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Voltei, agora é pra começar bem básico, mas, já vou avisando que nos próximos posts vou acelerar... 
Para os nossos estudos, levarei em consideração que você já tem acesso a um Banco na versão 11G preferencialmente. Usaremos o usuário (schema) HR como já mencionado.Select é o comando mais importante de todos os comandos da linguagem SQL. Sua funcionalidade pode ser extendida ao comandos update, insert e delete (veremos mais pra frente).
Sua sintaxe básica é select * from tabela.
Para ficar mais explicativo, veja o exemplo: select * from hr.employees;
* seleciona todos os campos da tabela.from: Obrigatório para informar de qual lugar serão pegos os dados.hr: É o nome do esquema (ou usuário, como queira chamar). Usando o hr. evita-se de acessar uma tabela com o mesmo nome só que de outro usuário e direciona à tabela correta. É uma boa prática usar o nome do esquema.Employees: É o nome da tabela.
Visualizaremos agora somente dois campos da tabela:
select e.first_name, e.last_namefrom hr.employees e;
Novos conceitos foram utilizados. Criamos um apelido (alias) para a tabela employees. Usamos a letra ‘e’ após o nome. Usamos normalmente nomes curtos ou letra como apelido. Isto facilita na hora de referenciar os campos. Além de novamente, ser uma boa prática.
O nome do primeiro campo é first_name, usamos o ‘e.’ na frente para referenciar a tabela correta. Acostume-se com essa prática, pois, quando estivermos programando com várias tabelas isso será muito útil.
select e.first_name nome, e.last_name as "Sobre nome"from hr.employees e;&nbsp; 
Agora, incorporamos os alias (apelidos) nas colunas (campo) de duas formas diferentes. Na primeira, usamos nome (letra minúscula) porém, aparece em letra maiúscula no resultado (não é aceito espaço em branco). Dessa forma, não faz diferença escreverem &nbsp;maiúsculo ou minúscula, o resultado será o mesmo. Na segunda forma, usamos o ‘as’ que o nome da coluna entre aspas dupla. Dessa forma, o que for escrito será mostrado da forma como é escrito, aceitando inclusive espaços em branco.
+ Tome cuidado na prova, é normal, aparecem questões&nbsp; na qual, a primeira forma é utilizada.
SELECT cust_last_name Name, cust_credit_limit"New Credit Limit" FROM customers;
Estarei no próximo post colocando mais comandos. Acelerando e incrementando mais comandos.