MySQL Tutorial/Select Query/User Defined Variables

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Create dynamic sql command

   <source lang="sql">

mysql> mysql> mysql> CREATE TABLE Employee(

   ->     id            int,
   ->     first_name    VARCHAR(15),
   ->     last_name     VARCHAR(15),
   ->     start_date    DATE,
   ->     end_date      DATE,
   ->     salary        FLOAT(8,2),
   ->     city          VARCHAR(10),
   ->     description   VARCHAR(15)
   -> );

Query OK, 0 rows affected (0.03 sec) mysql> mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->              values (1,"Jason",    "Martin",  "19960725",  "20060725", 1234.56, "Toronto",  "Programmer");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(2,"Alison",   "Mathews",  "19760321", "19860221", 6661.78, "Vancouver","Tester");

Query OK, 1 row affected (0.02 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(3,"James",    "Smith",    "19781212", "19900315", 6544.78, "Vancouver","Tester");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(4,"Celia",    "Rice",     "19821024", "19990421", 2344.78, "Vancouver","Manager");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(5,"Robert",   "Black",    "19840115", "19980808", 2334.78, "Vancouver","Tester");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(6,"Linda",    "Green",    "19870730", "19960104", 4322.78,"New York",  "Tester");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(7,"David",    "Larry",    "19901231", "19980212", 7897.78,"New York",  "Manager");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(8,"James",    "Cat",     "19960917",  "20020415", 1232.78,"Vancouver", "Tester");

Query OK, 1 row affected (0.00 sec) mysql> mysql> select * from Employee; +------+------------+-----------+------------+------------+---------+-----------+-------------+ | id | first_name | last_name | start_date | end_date | salary | city | description | +------+------------+-----------+------------+------------+---------+-----------+-------------+ | 1 | Jason | Martin | 1996-07-25 | 2006-07-25 | 1234.56 | Toronto | Programmer | | 2 | Alison | Mathews | 1976-03-21 | 1986-02-21 | 6661.78 | Vancouver | Tester | | 3 | James | Smith | 1978-12-12 | 1990-03-15 | 6544.78 | Vancouver | Tester | | 4 | Celia | Rice | 1982-10-24 | 1999-04-21 | 2344.78 | Vancouver | Manager | | 5 | Robert | Black | 1984-01-15 | 1998-08-08 | 2334.78 | Vancouver | Tester | | 6 | Linda | Green | 1987-07-30 | 1996-01-04 | 4322.78 | New York | Tester | | 7 | David | Larry | 1990-12-31 | 1998-02-12 | 7897.78 | New York | Manager | | 8 | James | Cat | 1996-09-17 | 2002-04-15 | 1232.78 | Vancouver | Tester | +------+------------+-----------+------------+------------+---------+-----------+-------------+ 8 rows in set (0.00 sec) mysql> mysql> mysql> mysql> mysql> SET @c = "first_name"; Query OK, 0 rows affected (0.00 sec) mysql> SET @s = CONCAT("SELECT ", @c, " FROM employee"); Query OK, 0 rows affected (0.00 sec) mysql> PREPARE stmt FROM @s; Query OK, 0 rows affected (0.00 sec) Statement prepared mysql> EXECUTE stmt; +------------+ | first_name | +------------+ | Jason | | Alison | | James | | Celia | | Robert | | Linda | | David | | James | +------------+ 8 rows in set (0.00 sec) mysql> DEALLOCATE PREPARE stmt; Query OK, 0 rows affected (0.00 sec) mysql> mysql> mysql> mysql> mysql> drop table Employee; Query OK, 0 rows affected (0.00 sec) mysql> mysql></source>


User variables are written as @var_name.

The variable name var_name may consist of alphanumeric characters from the current character set, ".","_", and $.

One way to set a user-defined variable is by issuing a SET statement:



   <source lang="sql">

SET @var_name = expr [, @var_name = expr] ...</source>


Use user-defined variable in the where statement

   <source lang="sql">

mysql> mysql> mysql> CREATE TABLE Employee(

   ->     id            int,
   ->     first_name    VARCHAR(15),
   ->     last_name     VARCHAR(15),
   ->     start_date    DATE,
   ->     end_date      DATE,
   ->     salary        FLOAT(8,2),
   ->     city          VARCHAR(10),
   ->     description   VARCHAR(15)
   -> );

Query OK, 0 rows affected (0.05 sec) mysql> mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->              values (1,"Jason",    "Martin",  "19960725",  "20060725", 1234.56, "Toronto",  "Programmer");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(2,"Alison",   "Mathews",  "19760321", "19860221", 6661.78, "Vancouver","Tester");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(3,"James",    "Smith",    "19781212", "19900315", 6544.78, "Vancouver","Tester");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(4,"Celia",    "Rice",     "19821024", "19990421", 2344.78, "Vancouver","Manager");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(5,"Robert",   "Black",    "19840115", "19980808", 2334.78, "Vancouver","Tester");

Query OK, 1 row affected (0.01 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(6,"Linda",    "Green",    "19870730", "19960104", 4322.78,"New York",  "Tester");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(7,"David",    "Larry",    "19901231", "19980212", 7897.78,"New York",  "Manager");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(8,"James",    "Cat",     "19960917",  "20020415", 1232.78,"Vancouver", "Tester");

Query OK, 1 row affected (0.00 sec) mysql> mysql> select * from Employee; +------+------------+-----------+------------+------------+---------+-----------+-------------+ | id | first_name | last_name | start_date | end_date | salary | city | description | +------+------------+-----------+------------+------------+---------+-----------+-------------+ | 1 | Jason | Martin | 1996-07-25 | 2006-07-25 | 1234.56 | Toronto | Programmer | | 2 | Alison | Mathews | 1976-03-21 | 1986-02-21 | 6661.78 | Vancouver | Tester | | 3 | James | Smith | 1978-12-12 | 1990-03-15 | 6544.78 | Vancouver | Tester | | 4 | Celia | Rice | 1982-10-24 | 1999-04-21 | 2344.78 | Vancouver | Manager | | 5 | Robert | Black | 1984-01-15 | 1998-08-08 | 2334.78 | Vancouver | Tester | | 6 | Linda | Green | 1987-07-30 | 1996-01-04 | 4322.78 | New York | Tester | | 7 | David | Larry | 1990-12-31 | 1998-02-12 | 7897.78 | New York | Manager | | 8 | James | Cat | 1996-09-17 | 2002-04-15 | 1232.78 | Vancouver | Tester | +------+------------+-----------+------------+------------+---------+-----------+-------------+ 8 rows in set (0.00 sec) mysql> mysql> SELECT @min_salary:=MIN(salary),@max_date:=MAX(start_date) FROM employee; +--------------------------+----------------------------+ | @min_salary:=MIN(salary) | @max_date:=MAX(start_date) | +--------------------------+----------------------------+ | 1232.78 | 1996-09-17 | +--------------------------+----------------------------+ 1 row in set (0.00 sec) mysql> mysql> SELECT * FROM employee WHERE salary=@min_salary OR start_date=@max_date; +------+------------+-----------+------------+------------+---------+-----------+-------------+ | id | first_name | last_name | start_date | end_date | salary | city | description | +------+------------+-----------+------------+------------+---------+-----------+-------------+ | 8 | James | Cat | 1996-09-17 | 2002-04-15 | 1232.78 | Vancouver | Tester | +------+------------+-----------+------------+------------+---------+-----------+-------------+ 1 row in set (0.00 sec) mysql> mysql> drop table Employee; Query OK, 0 rows affected (0.00 sec) mysql></source>


Using User-Defined Variables

User-defined variables are connection-specific.



   <source lang="sql">

mysql> mysql> mysql> CREATE TABLE Employee(

   ->     id            int,
   ->     first_name    VARCHAR(15),
   ->     last_name     VARCHAR(15),
   ->     start_date    DATE,
   ->     end_date      DATE,
   ->     salary        FLOAT(8,2),
   ->     city          VARCHAR(10),
   ->     description   VARCHAR(15)
   -> );

Query OK, 0 rows affected (0.03 sec) mysql> mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->              values (1,"Jason",    "Martin",  "19960725",  "20060725", 1234.56, "Toronto",  "Programmer");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(2,"Alison",   "Mathews",  "19760321", "19860221", 6661.78, "Vancouver","Tester");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(3,"James",    "Smith",    "19781212", "19900315", 6544.78, "Vancouver","Tester");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(4,"Celia",    "Rice",     "19821024", "19990421", 2344.78, "Vancouver","Manager");

Query OK, 1 row affected (0.02 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(5,"Robert",   "Black",    "19840115", "19980808", 2334.78, "Vancouver","Tester");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(6,"Linda",    "Green",    "19870730", "19960104", 4322.78,"New York",  "Tester");

Query OK, 1 row affected (0.02 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(7,"David",    "Larry",    "19901231", "19980212", 7897.78,"New York",  "Manager");

Query OK, 1 row affected (0.00 sec) mysql> mysql> insert into Employee(id,first_name, last_name, start_date, end_Date, salary, City, Description)

   ->               values(8,"James",    "Cat",     "19960917",  "20020415", 1232.78,"Vancouver", "Tester");

Query OK, 1 row affected (0.00 sec) mysql> mysql> select * from Employee; +------+------------+-----------+------------+------------+---------+-----------+-------------+ | id | first_name | last_name | start_date | end_date | salary | city | description | +------+------------+-----------+------------+------------+---------+-----------+-------------+ | 1 | Jason | Martin | 1996-07-25 | 2006-07-25 | 1234.56 | Toronto | Programmer | | 2 | Alison | Mathews | 1976-03-21 | 1986-02-21 | 6661.78 | Vancouver | Tester | | 3 | James | Smith | 1978-12-12 | 1990-03-15 | 6544.78 | Vancouver | Tester | | 4 | Celia | Rice | 1982-10-24 | 1999-04-21 | 2344.78 | Vancouver | Manager | | 5 | Robert | Black | 1984-01-15 | 1998-08-08 | 2334.78 | Vancouver | Tester | | 6 | Linda | Green | 1987-07-30 | 1996-01-04 | 4322.78 | New York | Tester | | 7 | David | Larry | 1990-12-31 | 1998-02-12 | 7897.78 | New York | Manager | | 8 | James | Cat | 1996-09-17 | 2002-04-15 | 1232.78 | Vancouver | Tester | +------+------------+-----------+------------+------------+---------+-----------+-------------+ 8 rows in set (0.00 sec) mysql> mysql> SELECT @min_salary:=MIN(salary),@max_date:=MAX(start_date) FROM employee; +--------------------------+----------------------------+ | @min_salary:=MIN(salary) | @max_date:=MAX(start_date) | +--------------------------+----------------------------+ | 1232.78 | 1996-09-17 | +--------------------------+----------------------------+ 1 row in set (0.00 sec) mysql> mysql> select @min_salary; +-----------------+ | @min_salary | +-----------------+ | 1232.7800292969 | +-----------------+ 1 row in set (0.00 sec) mysql> mysql> mysql> drop table Employee; Query OK, 0 rows affected (0.00 sec) mysql> mysql></source>


You can also assign a value to a user variable in statements other than SET

In this case, the assignment operator must be := and not = because = is treated as a comparison operator in non-SET statements:



   <source lang="sql">

mysql> mysql> SET @t1=0, @t2=1, @t3=2; Query OK, 0 rows affected (0.00 sec) mysql> mysql> SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3; +----------------------+------+------+------+ | @t1:=(@t2:=1)+@t3:=4 | @t1 | @t2 | @t3 | +----------------------+------+------+------+ | 5 | 5 | 1 | 4 | +----------------------+------+------+------+ 1 row in set (0.00 sec) mysql></source>