I mean something like:
create table Measures (
id_user int,
date timestamp,
measure_1 double default 'select measure_1 from Measures where data = '**/**/****'',
measure_2 double default 'select measure_1 from Measures where data = '**/**/****'');
In this way I insert the value of the last measure saved in the db..
Is it possible?
Not directly:
11.7 Data Type Default Values
... the default value must be a constant; it cannot be a function or an expression.
You'll have to do this on application level, or in a trigger as suggested by #Timekiller.
You can do that via a before-insert trigger.
Check if NEW.measure_1 is null, and if it is, then perform select and store results.
UPD:
Right, I was in a bit of a hurry yesterday, and forgot to give an example later. Trigger is a good replacement for complex default value - it will work transparently, will look just like the default value from database user standpoint, and you won't have to do anything on the application level, since triggers are stored in the database itself. It will look something like this:
CREATE TRIGGER `measures_bi_trigger` BEFORE INSERT ON `Measures`
FOR EACH ROW BEGIN
if NEW.measure_1 is null then
SET NEW.measure_1 = (select measure_1 from Measures where ... limit 1);
end if;
if NEW.measure_2 is null then
SET NEW.measure_2 = (select measure_2 from Measures where ... limit 1);
end if;
END
It's not exactly clear what should be in your where condition, so you'll have to substitute ... yourself. Note that your query should return exactly one row, so either use an aggregate function like MAX or order by ... limit 1. If your query returns no rows, NULL will be inserted.
Related
So I have table A which contains a column Z with default value set to string "diverse".
The queries come in from a php script which takes the data from a jqueryAJAX post request.
I would like to have my DB set the respective field of column Z to default value if the query received an empty string for insertion into/update of this column.
I really would like to accomplish this using mysql functionality, without using any more custom coded php logic.
I already read about using WHEN/THEN logic here:
Set default value if empty string is passed MySQL
and here
MySQL update CASE WHEN/THEN/ELSE
but these don't explain how I permanently configure a table/column in a way that it exposes this "default" behavior not just on receiving a NULL value, but also on receiving an empty string.
Besides:
If I set a column to NOT NULL and also add a default value for the column, would the query just fail if I tried to insert a/update to a NULL value, or would the DB instead flip to the default value?
MySQL/MariaDB will put the value to a column you specify in the insert/update-statement. If you do not provide a value, the default (if specified) will be used.
If you want to use the default value even if the insert/update-statement does provide a value (NULL / empty string), you will have to have the logic somewhere. The options are that you put the logic in your application code (PHP) or if you want to do it in MySQL/MariaDB, you can use a trigger to check the new value and act accordingly.
CREATE TRIGGER ins_A BEFORE INSERT ON A
FOR EACH ROW
BEGIN
IF NEW.Z is null or NEW.Z='' THEN
SET NEW.Z = 'diverse';
END IF;
END;
And do the same for UPDATE
Please follow 2 case bellow:
create table Test
(
a varchar(400) not null default 'test',
b varchar(10)
)
collate = utf8mb4_unicode_ci;
Case 1:
INSERT INTO Test (a, b) VALUES (NULL, 'case1');
=> resul: error because column a required not null;
Case 2:
INSERT INTO Test (b) VALUES ('case1');
=> OK, and result of column a = defaul value = test
Let's say a have a stored procedure SetCustomerName which has an input parameter Name, and I have a table customers with column Name.
So inside my stored procedure I want to set customer's name. If I write
UPDATE customers SET Name = Name;
this is incorrect and I see 2 other ways:
UPDATE customers SET Name = `Name`;
UPDATE customers SET customers.Name = Name;
First one works, but I didn't find in documentation that I can wrap parameters inside ` characters. Or did I miss it in the documentation (link is appreciated in this case).
What other ways are there and what is the standard way for such a case? Renaming input parameter is not good for me (because I have automatic object-relational mapping if you know what I mean).
UPDATE:
So, there is a link about backticks (http://dev.mysql.com/doc/refman/5.0/en/identifiers.html) but it's not explained deep enough how to use them (how to use them with parameters and column names).
And there is a very strange thing (at least for me): You can use backticks either way:
UPDATE customers SET Name = `Name`;
//or
UPDATE customers SET `Name` = Name;
//or even
UPDATE customers SET `Name` = `Name`;
and they all work absolutely the same way.
Don't you think this is strange? Is this strange behavior explained somewhere?
Simplest way to distinguished between your parameter and column (if both name is same) is to add table name in your column name.
UPDATE customers SET customers.Name = Name;
Even you can also add database prefix like
UPDATE yourdb.customers SET yourdb.customers.Name = Name;
By adding database name you can perform action on more than 1 database from single store procedure.
I think that your first example is actually backwards. If you're trying to set the "Name" column to the "Name" input parameter, I believe it should be:
UPDATE customers SET `Name` = Name;
And for the second example, you can set table aliases the same way that you do in all other statements:
UPDATE customers AS c SET c.Name = Name;
Not necessarily correct, but a fair way to better argument/parameter management, as well readability with easier understanding, especially while working with the SQL;
DROP PROCEDURE IF EXISTS spTerminalDataDailyStatistics; DELIMITER $$
CREATE PROCEDURE spTerminalDataDailyStatistics(
IN TimeFrom DATETIME,
IN DayCount INT(10),
IN CustomerID BIGINT(20)
) COMMENT 'Daily Terminal data statistics in a date range' BEGIN
# Validate argument
SET #TimeFrom := IF(TimeFrom IS NULL, DATE_FORMAT(NOW(), '%Y-%m-01 00:00:00'), TimeFrom);
SET #DayCount := IF(DayCount IS NULL, 5, DayCount);
SET #CustomerID := CustomerID;
# Determine parameter
SET #TimeTo = DATE_ADD(DATE_ADD(#TimeFrom, INTERVAL #DayCount DAY), INTERVAL -1 SECOND);
# Do the job
SELECT DATE_FORMAT(TD.TerminalDataTime, '%Y-%m-%d') AS DataPeriod,
COUNT(0) AS DataCount,
MIN(TD.TerminalDataTime) AS Earliest,
MAX(TD.TerminalDataTime) AS Latest
FROM pnl_terminaldata AS TD
WHERE TD.TerminalDataTime BETWEEN #TimeFrom AND #TimeTo
AND (#CustomerID IS NULL OR TD.CustomerID = #CustomerID)
GROUP BY DataPeriod
ORDER BY DataPeriod ASC;
END $$
DELIMITER ;
CALL spTerminalDataDailyStatistics('2021-12-01', 2, 1801);
Using backticks in MySQL query syntax is documented here:
http://dev.mysql.com/doc/refman/5.0/en/identifiers.html
So yes, your first example (using backticks) is correct.
Here is the link you are asking for:
http://dev.mysql.com/doc/refman/5.0/en/identifiers.html
The backticks are called "identifier quote" in MySql
I'm attempting to insert a datetime('2013-08-30 19:05:00') value into a SQL server database table column(smalldatetime) and the value stays "NULL" after the insert.
I'm doing this to 6 other columns that are the exact same type. What is this only occuring on one column? I've triple checked that the names of the columns are correct. Any ideas?
Assuming the situation is as you describe
CREATE TABLE T
(
S SMALLDATETIME NULL
)
INSERT INTO T
VALUES('2013-08-30 19:05:00')
SELECT *
FROM T /*Returns NULL*/
There are only two ways I can think of that this can happen.
1) That is an ambiguous datetime format. Under the wrong session options this won't cast correctly and if you have some additional options OFF it will return NULL rather than raise an error (e.g.)
SET LANGUAGE Italian;
SET ansi_warnings OFF;
SET arithabort OFF;
INSERT INTO T
VALUES('2013-08-30 19:05:00')
SELECT *
FROM T /*NULL inserted*/
2) You may have missed the column out in an INSTEAD OF trigger, or have an AFTER trigger that actually sets the value back to NULL.
How would one implement this in MySQL:
CREATE TABLE employee (
employeemonthly DECIMAL(10,2),
employeeyearly DECIMAL(10,2) DEFAULT employeemonthly*12
);
use a insert trigger for that. Something like this
DELIMITER |
CREATE TRIGGER default_yearly BEFORE INSERT ON employee
FOR EACH ROW BEGIN
SET NEW.employeeyearly = NEW.employeemonthly * 12;
END;
|
DELIMITER ;
I would use a view:
CREATE VIEW vemployees AS
SELECT e.employeemonthly,
e.employeemonthly * 12 AS employeeyearly
FROM EMPLOYEE e
...because there's little need to dedicate storage space for an easily calculated value. Otherwise, use a function or simply write the expression into whatever query/stored procedure you need.
What really depends is:
How often you need to access this data
How complex the operation is to get the result you need
I recommend starting with not storing the value. If performance gets to be a problem, then dedicate a column for the values storage -- not before. At that, a trigger is a bit overkill to me, when you can use (psuedocode):
INSERT INTO employee
(...employeemonthly, employeeyearly, ...)
VALUES
(...#employeemonthly, #employeemonthly * 12, ...
Use a trigger for the insert event, access the new record data using NEW and set the appropiate values.
Let's say a have a stored procedure SetCustomerName which has an input parameter Name, and I have a table customers with column Name.
So inside my stored procedure I want to set customer's name. If I write
UPDATE customers SET Name = Name;
this is incorrect and I see 2 other ways:
UPDATE customers SET Name = `Name`;
UPDATE customers SET customers.Name = Name;
First one works, but I didn't find in documentation that I can wrap parameters inside ` characters. Or did I miss it in the documentation (link is appreciated in this case).
What other ways are there and what is the standard way for such a case? Renaming input parameter is not good for me (because I have automatic object-relational mapping if you know what I mean).
UPDATE:
So, there is a link about backticks (http://dev.mysql.com/doc/refman/5.0/en/identifiers.html) but it's not explained deep enough how to use them (how to use them with parameters and column names).
And there is a very strange thing (at least for me): You can use backticks either way:
UPDATE customers SET Name = `Name`;
//or
UPDATE customers SET `Name` = Name;
//or even
UPDATE customers SET `Name` = `Name`;
and they all work absolutely the same way.
Don't you think this is strange? Is this strange behavior explained somewhere?
Simplest way to distinguished between your parameter and column (if both name is same) is to add table name in your column name.
UPDATE customers SET customers.Name = Name;
Even you can also add database prefix like
UPDATE yourdb.customers SET yourdb.customers.Name = Name;
By adding database name you can perform action on more than 1 database from single store procedure.
I think that your first example is actually backwards. If you're trying to set the "Name" column to the "Name" input parameter, I believe it should be:
UPDATE customers SET `Name` = Name;
And for the second example, you can set table aliases the same way that you do in all other statements:
UPDATE customers AS c SET c.Name = Name;
Not necessarily correct, but a fair way to better argument/parameter management, as well readability with easier understanding, especially while working with the SQL;
DROP PROCEDURE IF EXISTS spTerminalDataDailyStatistics; DELIMITER $$
CREATE PROCEDURE spTerminalDataDailyStatistics(
IN TimeFrom DATETIME,
IN DayCount INT(10),
IN CustomerID BIGINT(20)
) COMMENT 'Daily Terminal data statistics in a date range' BEGIN
# Validate argument
SET #TimeFrom := IF(TimeFrom IS NULL, DATE_FORMAT(NOW(), '%Y-%m-01 00:00:00'), TimeFrom);
SET #DayCount := IF(DayCount IS NULL, 5, DayCount);
SET #CustomerID := CustomerID;
# Determine parameter
SET #TimeTo = DATE_ADD(DATE_ADD(#TimeFrom, INTERVAL #DayCount DAY), INTERVAL -1 SECOND);
# Do the job
SELECT DATE_FORMAT(TD.TerminalDataTime, '%Y-%m-%d') AS DataPeriod,
COUNT(0) AS DataCount,
MIN(TD.TerminalDataTime) AS Earliest,
MAX(TD.TerminalDataTime) AS Latest
FROM pnl_terminaldata AS TD
WHERE TD.TerminalDataTime BETWEEN #TimeFrom AND #TimeTo
AND (#CustomerID IS NULL OR TD.CustomerID = #CustomerID)
GROUP BY DataPeriod
ORDER BY DataPeriod ASC;
END $$
DELIMITER ;
CALL spTerminalDataDailyStatistics('2021-12-01', 2, 1801);
Using backticks in MySQL query syntax is documented here:
http://dev.mysql.com/doc/refman/5.0/en/identifiers.html
So yes, your first example (using backticks) is correct.
Here is the link you are asking for:
http://dev.mysql.com/doc/refman/5.0/en/identifiers.html
The backticks are called "identifier quote" in MySql