_________________  .____     ___________________
 /   _____/\_____  \ |    |    \_   _____/\______ \
 \_____  \  /  / \  \|    |     |    __)_  |    |  \
 /        \/   \_/.  \    |___  |        \ |    `   \
/_______  /\_____\ \_/_______ \/_______  //_______  /
        \/        \__>       \/        \/         \/

. |03.04 - Querying the data I.                      .

   In  previous  chapters  we  created  a  table.  We
inserted data into it. This  time we will construct a
query to retrieve data from it.

  Let's begin with retrieval of all data presented in
the table:

  select * from users;

  This will give us following result:

    user_key  user_name    user_password
  ----------  -----------  ---------------
           1  user1        pass
           2  user2        pass
           3  user3        pass
           4  user4        pass

  Let's examine the query.

  SELECT is a keyword. Then there is star *. The star
is issued when we want to select all the columns from
the table.  Then there  is a FROM  keyword specified.
And the last word is the table name (users).

  Next  we will  try to  select only  certain columns
from the user table but still all rows:

  select user_name from users;

  give us:

  The database returns only  1 column (user_name) but
still all  the rows. What  if we would like  to limit
the result of the query to  only one row and e.g. two

  We would provide following query:

  select user_key, user_name
  from users
  where user_key = 1;

The result of the query would be following:

     user_key  user_name
  ----------  -----------
           1  user1

  In  the  above  SELECT  query we  added  the  WHERE
keyword. After this keyword we need to specify filter
clause  to tell  the  database what  rows  it has  to
return to us.

  WHERE  is  the  filter  keyword. And  then  in  the
expression user_key = 1  we specify selection of only
1  row (because  we have  user_key as  unique primary
key) and limit the results  to the row where user_key
equals 1.

  Let's say we would like  to explore the user's list
and  search  for  the  users  where  theirs  username
contains certain  string. This  could be  achieved by
the LIKE clause:

  select user_key, user_name
  from users
  where user_name like '%2';

The result would be:

    user_key  user_name
  ----------  -----------
           2  user2

  What we have specified here is the LIKE keyword and
specified  the filter  to  '%2'. That  would read  as
search  for rows  where  the  user_name contains  any
string from  the beginning but the  last character is
2. The  percentage sign  (%) translates here  to "any

  In the next chapter we will query some more data.

; v  | Thanks Linux and Vim | buy me a coffee   | o |;