MySQLUsing Variables

Setting Variables

Here are some ways to set variables:

  1. You can set a variable to a specific, string, number, date using SET

    EX: SET @var_string = 'my_var'; SET @var_num = '2' SET @var_date = '2015-07-20';

  2. you can set a variable to be the result of a select statement using :=

    EX: Select @var := '123'; (Note: You need to use := when assigning a variable not using the SET syntax, because in other statements, (select, update...) the "=" is used to compare, so when you add a colon before the "=", you are saying "This is not a comparison, this is a SET".)

  3. You can set a variable to be the result of a select statement using INTO

    (This was particularly helpful when I needed to dynamically choose which Partitions to query from)

EX: SET @start_date = '2015-07-20'; SET @end_date = '2016-01-31';

#this gets the year month value to use as the partition names
SET @start_yearmonth = (SELECT EXTRACT(YEAR_MONTH FROM @start_date));
SET @end_yearmonth = (SELECT EXTRACT(YEAR_MONTH FROM @end_date));

#put the partitions into a variable
SELECT GROUP_CONCAT(partition_name)
FROM information_schema.partitions p 
WHERE table_name = 'partitioned_table'
AND SUBSTRING_INDEX(partition_name,'P',-1) BETWEEN @start_yearmonth AND @end_yearmonth
INTO @partitions;

#put the query in a variable. You need to do this, because mysql did not recognize my variable as a variable in that position. You need to concat the value of the variable together with the rest of the query and then execute it as a stmt.
SET @query =
CONCAT('CREATE TABLE part_of_partitioned_table (PRIMARY KEY(id))
SELECT partitioned_table.*
FROM partitioned_table PARTITION(', @partitions,')
JOIN users u USING(user_id)
WHERE date(partitioned_table.date) BETWEEN ', @start_date,' AND ', @end_date);

#prepare the statement from @query
PREPARE stmt FROM @query;
#drop table
DROP TABLE IF EXISTS tech.part_of_partitioned_table;
#create table using statement
EXECUTE stmt;

Row Number and Group By using variables in Select Statement

Let's say we have a table team_person as below:

+======+===========+
| team |    person |
+======+===========+
|   A  |      John |
+------+-----------+
|   B  |     Smith |
+------+-----------+
|   A  |    Walter |
+------+-----------+
|   A  |     Louis |
+------+-----------+
|   C  | Elizabeth |
+------+-----------+
|   B  |     Wayne |
+------+-----------+


CREATE TABLE team_person AS SELECT 'A' team, 'John' person
UNION ALL  SELECT 'B' team,  'Smith' person
UNION ALL  SELECT 'A' team,  'Walter' person
UNION ALL  SELECT 'A' team,  'Louis' person
UNION ALL  SELECT 'C' team,  'Elizabeth' person
UNION ALL  SELECT 'B' team,  'Wayne' person;

To select the table team_person with additional row_number column, either

SELECT @row_no := @row_no+1 AS row_number, team, person
FROM team_person, (SELECT @row_no := 0) t;

OR

SET @row_no := 0;
SELECT  @row_no := @row_no + 1 AS row_number, team, person
FROM team_person;

will output the result below:

+============+======+===========+
| row_number | team |    person |
+============+======+===========+
|          1 |   A  |      John |
+------------+------+-----------+
|          2 |   B  |     Smith |
+------------+------+-----------+
|          3 |   A  |    Walter |
+------------+------+-----------+
|          4 |   A  |     Louis |
+------------+------+-----------+
|          5 |   C  | Elizabeth |
+------------+------+-----------+
|          6 |   B  |     Wayne |
+------------+------+-----------+

Finally, if we want to get the row_number group by column team

SELECT @row_no := IF(@prev_val = t.team, @row_no + 1, 1) AS row_number
   ,@prev_val := t.team AS team
   ,t.person  
FROM team_person t,
  (SELECT @row_no := 0) x,
  (SELECT @prev_val := '') y
ORDER BY t.team ASC,t.person DESC; 

+============+======+===========+
| row_number | team |    person |
+============+======+===========+
|          1 |   A  |    Walter |
+------------+------+-----------+
|          2 |   A  |     Louis |
+------------+------+-----------+
|          3 |   A  |      John |
+------------+------+-----------+
|          1 |   B  |     Wayne |
+------------+------+-----------+
|          2 |   B  |     Smith |
+------------+------+-----------+
|          1 |   C  | Elizabeth |
+------------+------+-----------+