sql fundamentals

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select

select command uses for accessing specific columns from table.

/* All data  */
select * from table_name;
/* specific columns data  */
select col_1, col_2, col_3 from table_name;

/* distinct / unique data */
select distinct col_1, col_2, col_3 from table_name;

where

where keyword with select for filtering records.


select col_1, col_2, col_3 from table_name
  where condition;

  • operators
operators intro
= equal
<> not equal
> greater than
< less than
>= greater than or equal
<= less than or equal
between mid point result
like searching purpose
in multiple possible value
and &&
or //

and / or / not

and / or / not use for sorting purpose

/* return where all condition is truty */
select col_1,col_2 from table_name
  where condition_1 and condtion_2;

/* return where any condition is truty */
select col_1,col_2 from table_name
  where condition_1 or condtion_2;

/* return all data without that condition */
select col_1,col_2 from table_name
  where not condition_1;
/* you can mixed and , or , not */

in + where

  • in
  • between
  • not between
select col_1,col_2,... from table_name
  where col_name in (values.....)
  /* between */
  where col_name between value_1 and value_2;

order by

order by key is so cool. By default data ordering by asc order.

  • asc (small from big)
  • desc (big from small)
select * from table_name
  where condition
  order by col_name desc , col_name desc, .... ;

searching tips

like / not like / wildcards

  • %a = end with a
  • a% = start with a
  • %a% = start or end with a
  • _a = find a at 2nd position
  • a_%_% = start with a and length is three
  • a%o = start with a and end with o
select col_1,col_2 from table_name
  where condition
  like '%sql';
  /* not like '%a' */

join

join keywords use for combining two or more tables.

  • join (inner)
  • left join
  • right join
  • full join

inner join

  • ignore data which left(joining table) any column is empty.
select table_1_or_2.any_coulmn,.... from table_1
  inner join table_2 on table_1.col_name = table_2.col_name;
  • hints
suppose,
 left = table_1
 right = table_2

left join

  • If left(joining table) any column is empty that time also shown data with null value.
select table_1_or_2.any_coulmn,.... from table_1
  left join table_2 on table_1.col_name = table_2.col_name;

Right join

  • If Right table has any empty column that time also shown data with null value.
select table_1_or_2.any_coulmn,.... from table_1
  right join table_2 on table_1.col_name = table_2.col_name;

Full join

  • Full join = left join + right join
  • Return all records although left or right table‚Äôs column has empty value .
select table_1_or_2.any_coulmn,.... from table_1
  full outer join table_2 on table_1.col_name = table_2.col_name;
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