vimwiki/tech/sql.wiki

= SQL =

Stuctered Query Language is a language that describes a method of fetching and
describing the relationship between different types of data

== Filtering data ==

To filter data we by some specifiers we use the `WHERE` clause. The where
clause can filter based on the default comparison operators. It can also do
string comparisons.

This system also works with dates, where all dates are in format `YYYY-MM-DD`.

=== IN ===

To see if an attribute is in a set we can use the `IN` keyword in conjunction
with a `WHERE` statement. For example,
`SELECT * FROM holidays WHERE Country IN ('spain', 'portugaul', 'USA');`

=== NOT ===

The `NOT` keyword can be placed before a conditional to specify that we want
everything that does NOT mean that condition.

=== LIKE ===

`LIKE` lets us filter on patterns. We can use the character `%` to specify a
wildcard, for any number of characters. We can use the character `_` to give an
exact number of wildcard characters we are looking for. For example,

`SELECT * FROM Bands WHERE BandName LIKE '%light%';`

will return all bands with the string 'light' in their name.

=== DISTINCT ===

`DISTINCT` filters results by only returning unique values in a column

== Views ==

A view can be created with the following command

{{{
CREATE VIEW [view_name] AS
SELECT column1, column2, ...
FROM table
WHERE condition;
  }}}

For example, this view returns all products witha price higher than the average
price of the table

{{{
CREATE VIEW [Products above average price] AS
SELECT Name, Price
FROM Products
WHERE Price > (SELECT AVG(Price) FROM Products)

SELECT * FROM [Prodcuts above average price]
  }}}

Views *can* be inserted into, you simply must specify the columns being
inserted into.

== Acess control commands ==

Two commands for managing access rights

* grant
  * used to grant one or more access rights or can be used to assign a user to
    a role
* revoke
  * revokes the acess rights

Typical access rights include

* select
* insert update
* delete
* references

== Also see ==

[[databases]]

[[index]]