I want to do something like this:
create table app_users
(
app_user_id smallint(6) not null auto_increment primary key,
api_key char(36) not null default uuid()
);
However this results in a error, is there a way to call a function for a default value in mysql?
thanks.
No, you can't.
However, you could easily create a trigger to do this, such as:
CREATE TRIGGER before_insert_app_users
BEFORE INSERT ON app_users
FOR EACH ROW
SET new.api_key = uuid();
As of mysql v8.0.13 it is possible to use an expression as a default value for a field:
The default value specified in a DEFAULT clause can be a literal constant or an expression. With one exception, enclose expression default values within parentheses to distinguish them from literal constant default values.
CREATE TABLE t1 (
uuid_field VARCHAR(32) DEFAULT (uuid()),
binary_uuid BINARY(16) DEFAULT (UUID_TO_BIN(UUID()))
);
As already stated you can't.
If you want to simulate this behavior you can use a trigger in this way:
CREATE TRIGGER before_insert_app_users
BEFORE INSERT ON app_users
FOR EACH ROW
IF new.uuid IS NULL
THEN
SET new.uuid = uuid();
END IF;
You still have to update previously existing rows, like this:
UPDATE app_users SET uuid = (SELECT uuid());
Unfortunately no, MySQL 5 requires constants for the default. The issue was discussed in much more detail in the link below. But the only answer is to allow null and add a table trigger.
MySQL only recently accepted UUID as part of their DB package, and it's not as feature rich as we'd like.
http://www.phpbuilder.com/board/showthread.php?t=10349169
I believe you can't:
the default value must be a constant; it cannot be a function or an expression
Note that MySQL's UUID() returns CHAR(36), and storing UUIDs as text (as shown in the other answers) is obviously inefficient. Instead, the column should be BINARY(16), and you can use UUID_TO_BIN() when inserting data and BIN_TO_UUID() when reading it back.
CREATE TABLE app_users
(
app_user_id SMALLINT(6) NOT NULL AUTO_INCREMENT PRIMARY KEY,
api_key BINARY(16)
);
CREATE TRIGGER before_insert_app_users
BEFORE INSERT ON app_users
FOR EACH ROW
IF new.api_key IS NULL
THEN
SET new.api_key = UUID_TO_BIN(UUID());
END IF;
Note that since MySQL doesn't really know this is a UUID, it can be difficult to troubleshoot problems with it stored as binary. This article explains how to create a generated column that will convert the UUID to text as needed without taking up any space or worrying about keeping separate binary and text versions in sync: https://mysqlserverteam.com/storing-uuid-values-in-mysql-tables/
In MariaDB starting from version 10.2.1 you can. See its documentation.
CREATE TABLE test ( uuid BINARY(16) PRIMARY KEY DEFAULT unhex(replace(uuid(),'-','')) );
INSERT INTO test () VALUES ();
SELECT * FROM test;
I'm not sure if the above answers are for an older version, but I saw somewhere that you can do this using the unhex() function. I tried it and it works. (maria db version 10.2)
You can do
.... column_name binary(16) not null default unhex(replace(uuid(),'-',''))
and it works. To see the uuid just do hex(column_name).
Harrison Fisk's answer was great when it was written, but now it's outdated.
Nowadays you can use an expression as a DEFAULT value. This is supported since MySQL 8.0 and MariaDB 10.2. Note that, if you're going to use non-deterministic functions like NOW() or USER(), you should not use binlog_format=statement.
Related
I want to do something like this:
create table app_users
(
app_user_id smallint(6) not null auto_increment primary key,
api_key char(36) not null default uuid()
);
However this results in a error, is there a way to call a function for a default value in mysql?
thanks.
No, you can't.
However, you could easily create a trigger to do this, such as:
CREATE TRIGGER before_insert_app_users
BEFORE INSERT ON app_users
FOR EACH ROW
SET new.api_key = uuid();
As of mysql v8.0.13 it is possible to use an expression as a default value for a field:
The default value specified in a DEFAULT clause can be a literal constant or an expression. With one exception, enclose expression default values within parentheses to distinguish them from literal constant default values.
CREATE TABLE t1 (
uuid_field VARCHAR(32) DEFAULT (uuid()),
binary_uuid BINARY(16) DEFAULT (UUID_TO_BIN(UUID()))
);
As already stated you can't.
If you want to simulate this behavior you can use a trigger in this way:
CREATE TRIGGER before_insert_app_users
BEFORE INSERT ON app_users
FOR EACH ROW
IF new.uuid IS NULL
THEN
SET new.uuid = uuid();
END IF;
You still have to update previously existing rows, like this:
UPDATE app_users SET uuid = (SELECT uuid());
Unfortunately no, MySQL 5 requires constants for the default. The issue was discussed in much more detail in the link below. But the only answer is to allow null and add a table trigger.
MySQL only recently accepted UUID as part of their DB package, and it's not as feature rich as we'd like.
http://www.phpbuilder.com/board/showthread.php?t=10349169
I believe you can't:
the default value must be a constant; it cannot be a function or an expression
Note that MySQL's UUID() returns CHAR(36), and storing UUIDs as text (as shown in the other answers) is obviously inefficient. Instead, the column should be BINARY(16), and you can use UUID_TO_BIN() when inserting data and BIN_TO_UUID() when reading it back.
CREATE TABLE app_users
(
app_user_id SMALLINT(6) NOT NULL AUTO_INCREMENT PRIMARY KEY,
api_key BINARY(16)
);
CREATE TRIGGER before_insert_app_users
BEFORE INSERT ON app_users
FOR EACH ROW
IF new.api_key IS NULL
THEN
SET new.api_key = UUID_TO_BIN(UUID());
END IF;
Note that since MySQL doesn't really know this is a UUID, it can be difficult to troubleshoot problems with it stored as binary. This article explains how to create a generated column that will convert the UUID to text as needed without taking up any space or worrying about keeping separate binary and text versions in sync: https://mysqlserverteam.com/storing-uuid-values-in-mysql-tables/
In MariaDB starting from version 10.2.1 you can. See its documentation.
CREATE TABLE test ( uuid BINARY(16) PRIMARY KEY DEFAULT unhex(replace(uuid(),'-','')) );
INSERT INTO test () VALUES ();
SELECT * FROM test;
I'm not sure if the above answers are for an older version, but I saw somewhere that you can do this using the unhex() function. I tried it and it works. (maria db version 10.2)
You can do
.... column_name binary(16) not null default unhex(replace(uuid(),'-',''))
and it works. To see the uuid just do hex(column_name).
Harrison Fisk's answer was great when it was written, but now it's outdated.
Nowadays you can use an expression as a DEFAULT value. This is supported since MySQL 8.0 and MariaDB 10.2. Note that, if you're going to use non-deterministic functions like NOW() or USER(), you should not use binlog_format=statement.
Im revisiting my database and noticed I had some primary keys that were of type INT.
This wasn't unique enough so I thought I would have a guid.
I come from a microsoft sql background and in the ssms you can
choose type to "uniqeidentifier" and auto increment it.
In mysql however Ive found that you have to make triggers that execute on insert for the tables you want
to generate a guide id for. Example:
Table:
CREATE TABLE `tbl_test` (
`GUID` char(40) NOT NULL,
`Name` varchar(50) NOT NULL,
PRIMARY KEY (`GUID`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
Trigger:
CREATE TRIGGER `t_GUID` BEFORE INSERT ON `tbl_test`
FOR EACH ROW begin
SET new.GUID = uuid();
Alternatively you have to insert the guid yourself in the backend.
Im no DB expert but still remember that triggers cause performance problems.
The above is something I found here and is 9 years old so I was hoping something has changed?
As far as stated in the documentation, you can use uid() as a column default starting version 8.0.13, so something like this should work:
create table tbl_test (
guid binary(16) default (uuid_to_bin(uuid())) not null primary key,
name varchar(50) not null
);
This is pretty much copied from the documentation. I don't have a recent enough version of MySQL at hand to test this.
You can make a
INSERT INTO `tbl_test` VALUES (uuid(),'testname');
This would generate a new uuid, when you call it.
Or you can also use the modern uuid v4 by using one of these functions instead of the standard uuid(), which is more random than the uuid in mysql
How to generate a UUIDv4 in MySQL?
You can use since 8.0.13
CREATE TABLE t1 (
uuid_field VARCHAR(40) DEFAULT (uuid())
);
But you wanted more than unique, but here are only allowed internal functions and not user defined as for uuid v4, for that uyou need the trogger
As per the documentation, BINARY(x) adds some hidden padding bytes to the end of each entry, & VARCHAR(40) also wastes space by not being encoded directly in binary. Using VARBINARY(16) would be more efficient.
Also, more entropy (unguessability / security) per byte is available from RANDOM_BYTES(16) than standardized UUIDs, because they use some sections to encode constant metadata.
Perhaps the below will work for your needs.
-- example
CREATE TABLE `tbl_test` (
`GUID` VARBINARY(16) DEFAULT (RANDOM_BYTES(16)) NOT NULL PRIMARY KEY,
`Name` VARCHAR(50) NOT NULL
);
Im revisiting my database and noticed I had some primary keys that were of type INT.
This wasn't unique enough so I thought I would have a guid.
I come from a microsoft sql background and in the ssms you can
choose type to "uniqeidentifier" and auto increment it.
In mysql however Ive found that you have to make triggers that execute on insert for the tables you want
to generate a guide id for. Example:
Table:
CREATE TABLE `tbl_test` (
`GUID` char(40) NOT NULL,
`Name` varchar(50) NOT NULL,
PRIMARY KEY (`GUID`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
Trigger:
CREATE TRIGGER `t_GUID` BEFORE INSERT ON `tbl_test`
FOR EACH ROW begin
SET new.GUID = uuid();
Alternatively you have to insert the guid yourself in the backend.
Im no DB expert but still remember that triggers cause performance problems.
The above is something I found here and is 9 years old so I was hoping something has changed?
As far as stated in the documentation, you can use uid() as a column default starting version 8.0.13, so something like this should work:
create table tbl_test (
guid binary(16) default (uuid_to_bin(uuid())) not null primary key,
name varchar(50) not null
);
This is pretty much copied from the documentation. I don't have a recent enough version of MySQL at hand to test this.
You can make a
INSERT INTO `tbl_test` VALUES (uuid(),'testname');
This would generate a new uuid, when you call it.
Or you can also use the modern uuid v4 by using one of these functions instead of the standard uuid(), which is more random than the uuid in mysql
How to generate a UUIDv4 in MySQL?
You can use since 8.0.13
CREATE TABLE t1 (
uuid_field VARCHAR(40) DEFAULT (uuid())
);
But you wanted more than unique, but here are only allowed internal functions and not user defined as for uuid v4, for that uyou need the trogger
As per the documentation, BINARY(x) adds some hidden padding bytes to the end of each entry, & VARCHAR(40) also wastes space by not being encoded directly in binary. Using VARBINARY(16) would be more efficient.
Also, more entropy (unguessability / security) per byte is available from RANDOM_BYTES(16) than standardized UUIDs, because they use some sections to encode constant metadata.
Perhaps the below will work for your needs.
-- example
CREATE TABLE `tbl_test` (
`GUID` VARBINARY(16) DEFAULT (RANDOM_BYTES(16)) NOT NULL PRIMARY KEY,
`Name` VARCHAR(50) NOT NULL
);
I am a user of a some host company which serves my MySql database. Due to their replication problem, the autoincrement values increses by 10, which seems to be a common problem.
My question is how can I simulate (safely) autoincrement feature so that the column have an consecutive ID?
My idea was to implement some sequence mechanism to solve my problem, but I do not know if it is a best option. I had found such a code snipset over the web:
DELIMITER ;;
DROP TABLE IF EXISTS `sequence`;;
CREATE TABLE `sequence` (
`name` CHAR(16) NOT NULL,
`value` BIGINT UNSIGNED NOT NULL,
PRIMARY KEY (`name`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1;;
DROP FUNCTION IF EXISTS `nextval`;
CREATE FUNCTION `nextval`(thename CHAR(16) CHARSET latin1)
RETURNS BIGINT UNSIGNED
MODIFIES SQL DATA
SQL SECURITY DEFINER
BEGIN
INSERT INTO `sequence`
SET `name`=thename,
`value`=(#val:=##auto_increment_offset)+##auto_increment_increment
ON DUPLICATE KEY
UPDATE `value`=(#val:=`value`)+##auto_increment_increment;
RETURN #val;
END ;;
DELIMITER ;
which seems quite all correct. My second question is if this solution is concurrent-safe? Of course INSERT statement is, but what about ON DUPLICATE KEY update?
Thanks!
Why do you need to have it in the first place?
Even with auto_increment_increment == 1 you are not guaranteed, that the autoincrement field in the table will have consecutive values (what if the rows are deleted, hmm?).
With autoincrement you are simply guaranteed by the db engine, that the field will be unique, nothing else, really.
EDIT: I want to reiterate: In my opinion, it is not a good idea to assume things like concurrent values of an autoincrement column, because it is going to bite you later.
EDIT2: Anyway, this can be "solved" by an "on insert" trigger
create trigger "sequence_b_ins" before insert on `sequence`
for each row
begin
NEW.id = select max(id)+1 from `sequence`;
end
Or something along these lines (sorry, not tested)
Another option would be to use a stored proc to do the insert and have it either select max id from your table or keep another table with the current id being used and update as id's are used.
I want to do something like this:
create table app_users
(
app_user_id smallint(6) not null auto_increment primary key,
api_key char(36) not null default uuid()
);
However this results in a error, is there a way to call a function for a default value in mysql?
thanks.
No, you can't.
However, you could easily create a trigger to do this, such as:
CREATE TRIGGER before_insert_app_users
BEFORE INSERT ON app_users
FOR EACH ROW
SET new.api_key = uuid();
As of mysql v8.0.13 it is possible to use an expression as a default value for a field:
The default value specified in a DEFAULT clause can be a literal constant or an expression. With one exception, enclose expression default values within parentheses to distinguish them from literal constant default values.
CREATE TABLE t1 (
uuid_field VARCHAR(32) DEFAULT (uuid()),
binary_uuid BINARY(16) DEFAULT (UUID_TO_BIN(UUID()))
);
As already stated you can't.
If you want to simulate this behavior you can use a trigger in this way:
CREATE TRIGGER before_insert_app_users
BEFORE INSERT ON app_users
FOR EACH ROW
IF new.uuid IS NULL
THEN
SET new.uuid = uuid();
END IF;
You still have to update previously existing rows, like this:
UPDATE app_users SET uuid = (SELECT uuid());
Unfortunately no, MySQL 5 requires constants for the default. The issue was discussed in much more detail in the link below. But the only answer is to allow null and add a table trigger.
MySQL only recently accepted UUID as part of their DB package, and it's not as feature rich as we'd like.
http://www.phpbuilder.com/board/showthread.php?t=10349169
I believe you can't:
the default value must be a constant; it cannot be a function or an expression
Note that MySQL's UUID() returns CHAR(36), and storing UUIDs as text (as shown in the other answers) is obviously inefficient. Instead, the column should be BINARY(16), and you can use UUID_TO_BIN() when inserting data and BIN_TO_UUID() when reading it back.
CREATE TABLE app_users
(
app_user_id SMALLINT(6) NOT NULL AUTO_INCREMENT PRIMARY KEY,
api_key BINARY(16)
);
CREATE TRIGGER before_insert_app_users
BEFORE INSERT ON app_users
FOR EACH ROW
IF new.api_key IS NULL
THEN
SET new.api_key = UUID_TO_BIN(UUID());
END IF;
Note that since MySQL doesn't really know this is a UUID, it can be difficult to troubleshoot problems with it stored as binary. This article explains how to create a generated column that will convert the UUID to text as needed without taking up any space or worrying about keeping separate binary and text versions in sync: https://mysqlserverteam.com/storing-uuid-values-in-mysql-tables/
In MariaDB starting from version 10.2.1 you can. See its documentation.
CREATE TABLE test ( uuid BINARY(16) PRIMARY KEY DEFAULT unhex(replace(uuid(),'-','')) );
INSERT INTO test () VALUES ();
SELECT * FROM test;
I'm not sure if the above answers are for an older version, but I saw somewhere that you can do this using the unhex() function. I tried it and it works. (maria db version 10.2)
You can do
.... column_name binary(16) not null default unhex(replace(uuid(),'-',''))
and it works. To see the uuid just do hex(column_name).
Harrison Fisk's answer was great when it was written, but now it's outdated.
Nowadays you can use an expression as a DEFAULT value. This is supported since MySQL 8.0 and MariaDB 10.2. Note that, if you're going to use non-deterministic functions like NOW() or USER(), you should not use binlog_format=statement.