I was wanting a mysql trigger for a specific field. When a NULL value is inserted into this field I would like to replace it with a default value. I know this code is not correct, but it would give you an idea of what I want.
CREATE TABLE IF NOT EXISTS example(
id INT NOT NULL AUTO_INCREMENT,
parent_id INT NOT NULL
);
CREATE TRIGGER insert_parentId_trigger BEFORE INSERT ON example
FOR EACH ROW BEGIN
IF parent_id = NULL THEN
SET parent_id = 0;
END IF
END;
Declare that column with not null default 0 instead of using trigger later.
When mysql is already handling that condition why we need to use trigger here?
You can use alter command to update your definition of your column
ALTER TABLE example MODIFY parent_id INT(11) NOT NULL DEFAULT 0
I agree with the example of a Null be simplistically set to 0, then using a "Default" is fine, but what if you're after a value that's variable and cannot be a Default? For example:
IF `Creator` Is NULL THEN
SET `Creator` = current_user();
END IF
For this type of use case, or something that requires a lookup, you will not be able to use Default.
Besides that Shakti Singh is totally right with the default value, you have to compare NULL values with IS instead of =
It should be IF parent_id IS NULL
Related
ALTER TABLE `pages` MODIFY `views` INT(11) NOT NULL DEFAULT 0
Trying to alter a column in a table which is currently allows NULL and has no default.
I want it to be NOT NULL and have a default of 0.
The Error message I get is
Invalid use of NULL value
This is because the table already has a row or more with null value, you might need to update those to 0 before executing ALTER table, e.g.:
UPDATE test SET views = 0 WHERE views IS NULL;
ALTER TABLE test MODIFY COLUMN views int NOT NULL DEFAULT 0;
Here's the SQL Fiddle (commenting out update statement will result in the same error).
I want to create a column with default value as null and when any operation is performed it should change to 0. How do i do this in mysql database?
Here example how to add colum in existing table with default value
ALTER TABLE `test1` ADD `no` INT NULL DEFAULT NULL ;
When you call function then you have to write following query
UPDATE test1 SET `no` = '0' WHERE `test1`.`id` =your_id;
CREATE TABLE test
(
id INT NOT NULL AUTO_INCREMENT,
PRIMARY KEY(id),
test_id INT,
cost FLOAT(5,2) DEFAULT NULL,
);
each time when you do some operation on that you need to update it as #Sadikhasan
or write a trigger that will update it to zero automatically.
if the operation you want to perform is read then write trigger on ON SELECT
if the operation you want to perform is update then write trigger on ON UPDATE
like wise for others.
Can we specify a column in mysql as "not empty" / "required". The requirement is to ensure that the field never remains blank on any record insertion.
I assume you don't want blank (empty string, as opposed to NULL) values to be allowed in the table either.
Normally, that's what a CHECK constraint for. You do something like
CREATE TABLE
mytable
(
myfield NOT NULL VARCHAR(200),
CHECK(myfield > '')
)
However, MySQL parses the constraint but does not enforce it. You are still allowed to insert empty values.
To work around that, create a BEFORE INSERT trigger and raise a signal on an attempt to insert a blank value:
CREATE TRIGGER
tr_mytable_bi
BEFORE INSERT
ON mytable
FOR EACH ROW
BEGIN
IF NEW.myfield = '' THEN
SIGNAL SQLSTATE '45001' SET MESSAGE_TEXT = 'Blank value on mytable.myfield';
END IF;
END;
Do the same on BEFORE UPDATE if you want to forbid updates to a blank value as well.
MySQL8 now has value constraints, which allows you to implement value restrictions without using triggers
For example:
CREATE TABLE my_table (
myfield VARCHAR(255) NOT NULL
CONSTRAINT myfield_not_empty CHECK(
LENGTH(myfield) > 0
)
);
Will ensure the values in myfield will never be empty
Use NOT NULL.
It causes input to be required.
But a field to be not empty you need validation script explicitly.
Example:
create table example(
login_name varchar(16) not null
pass_word varchar(32) not null
);
Refer to: Wiki on Null (SQL)
You can define the column as NOT NULL as answered by #Ravinder
In addition to it you can set a default value to a column. If we take the example of previous answer, we can do it as below:
create table example(
login_name varchar(16) = null
pass_word varchar(32) = null
);
Here if no value is received for these columns then NULL will be inserted by default.
I am trying to figure out make a trigger to assign the value of the auto incremented 'ID' primary key field that is auto generated upon insert to another field 'Sort_Placement' so they are the same after insert.
If you are wondering why I am doing this, 'Sort_Placement' is used as a sort value in a table that can be changed but by default the record is added to the bottom of the table
Table Data
`ID` mediumint(8) unsigned NOT NULL AUTO_INCREMENT,
`Account_Num` mediumint(8) unsigned NOT NULL,
`Product_Num` mediumint(8) unsigned NOT NULL,
`Sort_Placement` mediumint(8) unsigned DEFAULT NULL,
`Order_Qty_C` smallint(6) NOT NULL DEFAULT '0',
`Order_Qty_B` smallint(6) NOT NULL DEFAULT '0',
`Discount` decimal(6,2) NOT NULL DEFAULT '0.00',
PRIMARY KEY (`ID`),
UNIQUE KEY `ID_UNIQUE` (`ID`)
After Insert Trigger
CREATE
TRIGGER `order_guide_insert_trigger`
AFTER INSERT ON `order_guide`
FOR EACH ROW
BEGIN
IF Sort_Placement IS NULL THEN
SET Sort_Placement = NEW.ID;
END IF;
END;
I have tried a bunch of combinations of using the "NEW" prefix with no luck. For example putting the NEW prefix before each field name.
Trying it out
INSERT INTO `order_guide` (`Account_Num`, `Product_Num`) VALUES ('5966', '3');
Insert Error
ERROR 1054: Unknown column 'Sort_Placement' in 'field list'
This seems like a bit of a hack job but I was able to get it working using the LAST_INSERT_ID() function built into MySQL.
CREATE TRIGGER `order_guide_insert_trigger`
BEFORE INSERT ON `order_guide`
FOR EACH ROW
BEGIN
IF NEW.Sort_Placement IS NULL THEN
SET NEW.Sort_Placement = LAST_INSERT_ID() + 1;
END IF;
END;
This also works and seems to work
CREATE TRIGGER `order_guide_insert_trigger`
BEFORE INSERT ON `order_guide`
FOR EACH ROW
BEGIN
IF NEW.Sort_Placement IS NULL THEN
SET NEW.Sort_Placement = (SELECT ID FROM order_Guide ORDER BY id DESC LIMIT 1) + 1;
END IF;
END;
I ran into a similar (yet different) requirement, where a field value in the table needed to be based on the new record's Auto Increment ID. I found two solutions that worked for me.
The first option was to use an event timer that runs every 60 seconds. The event updated the records where my field was set to the default of null. Not a bad solution if you don't mind the up to 60 second delay (you could run it every 1 second if the field that is being update is indexed). Basically the event does this:
CREATE EVENT `evt_fixerupper`
ON SCHEDULE EVERY 1 MINUTE
ENABLE
COMMENT '' DO
BEGIN
UPDATE table_a SET table_a.other_field=CONCAT(table_a.id,'-kittens')
WHERE ISNULL(table_a.other_field);
END;
The other option was to generate my own unique primary IDs (rather than relying upon AUTOINCREMENT. In this case I used a function (in my application) modeled after the perl module https://metacpan.org/pod/Data::Uniqid. the generated ID's are huge in length, but they work well, and I know the value before I insert, so I can use it to generate values for additional fields.
I'm trying to set up a table row that does not need a value (no error message if empty) but will insert a default value if empty, as in:
ALTER TABLE tablename ALTER rowname TINYINT UNSIGNED NOT NULL;
ALTER TABLE tablename ALTER rowname SET DEFAULT 0;
I set it up this way, but the value inserted is still NULL.
Is this even possible?
Thanks!
The NOT NULL clearly says the value for this column cannot be NULL. So there is no NULL value used for the column. But it can hold the integer value 0 as it is a valid integer value.
It is a bit strange. Given this table definition:
create table def ( i int not null default 0 );
This statement works:
insert into def () values ();
select * from def;
+---+
| i |
+---+
| 0 |
+---+
But this statement generates an error:
insert into def values( null );
ERROR 1048 (23000): Column 'i' cannot be null
So it seems that "not null default 0" will supply the default of 0 when the column is not explicitly assigned a value (which would normally be assigned a NULL), but does not allow NULL to be explicitly assigned because of the NOT NULL attribute.
Are you sure that the value being inserted into the database is actually NULL, and that your programming language isn't just disregarding type and treating a 0 in the row as equivalent to NULL? If your column is defined as NOT NULL, MySQL shouldn't even be allowing NULL at all, let alone use it as the default.