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 of a column.
Therefore, we can only be used to return a single column.

{{{
SELECT DISTINCT nationality FROM users;
  }}}

== Presentation ==

=== Order by ===

`ORDER BY` allows us to descide how the query will be shown. For example,

{{{
SELECT Firstname, Lastname
FROM users
ORDER BY Lastname [SORT];
  }}}

where `[SORT]` is

* DESC
  * descending order
* ASC
  * ascending order

=== CASE ===

Case is a statement block where we can choose exactly what text is returned
based on values in the database.

{{{
SELECT Name,
CASE
    WHEN Country = 'USA' THEN 'North America'
    WHEN Country = 'UK' THEN 'Europe'
    ELSE 'Wolrd'
END
FROM Users;
  }}}

=== LIMIT ===

`LIMIT` allows us to retstrict the number or rows returned.

`SELECT * FROM users ORDER BY name DESC LIMIT 1`

=== COUNT ===

If we want just the number of rows, we can use count.

`SELECT COUNT(*) FROM users WHERE dob > '2000-01-01'`

=== math functions ===

If we want to sum the values in ONE column we can use sum

`SELECT SUM(cash_balance) FROM users`

If we want to find an average,

`SELECT AVG(price) FROM products`

This same principle applies for

* `MAX`
* `MIN`

=== GROUP BY ===

`GROUP BY` allows us to combine with the above math and `COUNT` functions by
grouping totals based upon some other row.

`SELECT Team, COUNT(*) FROM Players GROUP BY Team`

=== JOIN ===

`JOIN` allows two tables to be returned together, where they are 'joined' upon
a shared field.

{{{
SELECT Orders.ID, Customers.Name, Orders.Name
FROM Orders
INNER JOIN Customers ON Orders.CustomerId=Customers.Id
  }}}

There are four types of joins
* `INNER JOIN` returns all records that having matching records in both tables
* `LEFT OUTER JOIN` returns all records from the left (Orders in above example)
  table, and the matched records from the right table
* `RIGHT OUTER JOIN` returns all records from the right (Customers in above
  example) table, and the matched records from the left table
* `FULL OUTER JOIN` returns all records when there is a match in either the
  left or right table

== 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]]