MySQL
Usando variables

Variables de configuración

Aquí hay algunas maneras de establecer variables:

1. Puede configurar una variable para una cadena, número, fecha específica usando SET

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

2. puede configurar una variable para que sea el resultado de una instrucción de selección usando: =

   EX: Seleccione @var: = '123'; (Nota: debe usar: = cuando asigne una variable que no use la sintaxis SET, porque en otras declaraciones, (seleccione, actualice ...) se usa el "=" para comparar, por lo que cuando agregue dos puntos antes de " = ", estás diciendo" Esto no es una comparación, esto es un SET ".)

3. Puede configurar una variable para que sea el resultado de una instrucción de selección utilizando INTO

   (Esto fue particularmente útil cuando necesitaba elegir dinámicamente de qué particiones consultar)

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;

Número de fila y grupo utilizando variables en la instrucción Select

Digamos que tenemos una mesa team_person como abajo:

+======+===========+
| 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;

Para seleccionar la tabla team_person con columna adicional row_number , ya sea

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

O

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

Saldrá el resultado a continuación:

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

Finalmente, si queremos obtener el grupo row_number por team columna

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 |
+------------+------+-----------+