Sql queries

Example

SELECT *
FROM  users
WHERE username = "dvader"
AND   age > 40;

Query Components

SELECT

Determines what columns you want to return.

SELECT *            # select all columns
SELECT col1, col2   # select col1, and col2 only
SELECT col1 AS foo  # rename column in results table

DISTINCT

Distinct only returns one result per unique value of N

SELECT DISTINCT username FROM  users WHERE last_name = "skywalker";

CONCAT

CONCAT lets you combine columns into one

SELECT CONCAT(first_name, " ", last_name) AS fullname FROM users;

SUBSTRING_INDEX

SUBSTRING_INDEX lets you split/tokenize a string

SELECT SUBSTRING_INDEX('www.mysql.com', '.', 2)
##> www.mysql

LIMIT

LIMIT lets you choose only the first N results

SELECT * FROM users LIMIT 10

ORDER BY

SELECT * FROM users ORDER BY first_name ASC;

JOINS

See sql: joins

Techniques

Nested Queries

SELECT user_Id
FROM (
   SELECT DISTINCT     d.user_Id                                           ,
                       CONCAT(u.firstName, " ", u.lastName)  user_fullName
   FROM                userDepartmentTable             d
   INNER JOIN          userTable AS u ON d.user_Id  =  u.user_Id
   WHERE               d.department_Id              =  2
) u
WHERE u.user_fullName REGEXP 'andr%';

Dynamically Defined Tables

Some databases do not implement the SQL IN operator.
Instead, you can use VALUES.

SELECT *
FROM (
  VALUES (1), (2)
) foo(id);

Most useful within a join

SELECT *
FROM (
  VALUES (1), (2)
) foo(id)

INNER JOIN (
  (VALUES (1)) bar(id)
) ON foo.id = bar.id