SQL SELECT

Internal

• SQL
• SQL WHERE

Overview

A query consists in at least one (SELECT) and at most six categories of clauses:

SELECT [one or more things] 
FROM [one or more places] 
WHERE [one or more conditions apply] 
GROUP BY [...] HAVING [...]
ORDER BY [...];

Almost every query will include at least three of these clauses (SELECT, FROM and WHERE).

Once submitted to the server, the server verifies the syntax and passes the query to the query optimizer to be executed. The optimizer look at such things as the order in which to join tables, what indexes are available, etc. and the picks an execution plan.

The execution consists in selecting all rows, possibly across multiple tables, then filtering and discarding the rows that do not match the filter conditions specified in the WHERE clause, if present.

Upon execution, an SQL query returns a result set, which is just another table containing rows and columns.

Example

SELECT id, name FROM person WHERE id = 1;

The following query:

SELECT;

is valid, it returns one empty row.

Clauses

SELECT

SELECT [one or more things] ...

SELECT determines which columns from the FROM sources need to be retrieved and included in the result set. Even though SELECT clause is the first in the statement, it is one of the last clauses to be evaluated. This is because before the server can determine what to include in the final result set, it needs to know all of the possible columns that could be included, which are determined by the content of the FROM clause. A very obvious example is SELECT * FROM person;: only the person table columns can be included.

All columns can be specified with an *:

SELECT * FROM person

Specific columns can be named:

SELECT id, name FROM person

Aside from column names, SELECT accepts literals, such as numbers or strings, expressions, built-in function calls and user-defined function calls. In each of these cases, the label of the "synthetic" column such generated can be specified after the literal/expression/function. These labels are known as column aliases:

/* prefix, shifted_id and upper_name are column aliases */
SELECT name original_name, '@@@' some_prefix, id + 5 shifted_id, UPPER(name) upper_name FROM person

 original_name | some_prefix | shifted_id |  upper_name
---------------+-------------+------------+--------------
 Binh Ngo Jr.  | @@@         |          6 | BINH NGO JR.

In order to make your column aliases stand out even more, the AS keyword can be optionally used before the alias name:

SELECT name AS original_name, '@@@' AS some_prefix, id + 5 AS shifted_id, UPPER(name) AS upper_name FROM person

DISTINCT

The DISTINCT keyword specified after SELECT removes duplicates.

SELECT DISTINCT eye_color FROM person;

Note that generating a distinct set of results requires data to be sorted, which may have performance implication for large result sets.

Analyze situations. What if there are multiple columns?

FROM

FROM [one or more places]

FROM identifies the tables from which to retrieve data, and how the tables should be joined.

The "places" we select from can be:

• permanent
• derived
• temporary
• virtual(view)

Table alias.

Querying Multiple Tables

Querying Multiple Tables

WHERE

The WHERE Clause

GROUP BY ... HAVING

GROUP BY ... HAVING

ORDER BY

ORDER BY clause sorts the rows of the final result set by one or more columns.