Are there solutions to maintain a VARCHAR(65535) field unique per ID in MySQL?
I would use a hashing function on field's value and store it in a separate unique field on the same table.
So your table would be something like this:
create table my_table (
id integer not null primary key,
text_content varchar(65535) not null,
text_hash varchar(128) not null unique
);
When inserting on that table, you will compute sha256 hash for text_content field and store it in text_hash field. That way you can be pretty sure text_content values are unique on your table.
If you like DB SIDE programming, you can put this logic on a trigger provided MySQL supports them.
Related
How can I insert new data in column after adding column without using update function. for example
"alter table Employee add column Gender varchar(1) after Birthdate then I get wrong when I used this statement insert into Employee(ENumber,EmpName,Birthdate,Address,Salary,DNumber,Gender)
-> values
-> ('E001','GSInocencio','1988-01-15','Munoz',18000,'D005','F'),
It gives me error Duplicate entry 'E001' for key 'PRIMARY'
MariaDB [Employees_Valdez]>
The messages is pretty clear: You already have an employee with that ENumber value.
You have a UNIQUE constraint on that column, it's a PRIMARY KEY, so either pick a different value, or use a different primary key.
One thing to note is MySQL doesn't use complex string primary keys very efficiently, they're also a real hassle for relating data since they're so big. It's usually better to include a standard id INT AUTO_INCREMENT PRIMARY KEY column and then have things like ENumber being a secondary UNIQUE constraint.
You can then relate data using the 4-byte id value, or 8-byte if BIGINT is a concern like you might have two billion employees.
Is it possible to create auto-increment based on a specific field? For example i have UserId and Status fields, so for each row with same UserId i need to auto-increment its Status, not global.
There is three thing that come to mind when I read your question. One was an auto incrementing field which acts as your ID number. Updating a table with data that has no unique ID number. Searching for fields with the same Userid to Status
Mt First example is of a creating a table and your AUTO_INCREMENTing number ID:
CREATE TABLE tableNameHere
(
UniqueID int NOT NULL AUTO_INCREMENT,
FirstName varchar(255) NOT NULL,
StatusOrYourColumn int(100) NOT NULL,
PRIMARY KEY (UniqueUD)
)
More on auto incrementation.
You may have already built your table and now want to 'add' additional and or modify your fields using ALTER:
ALTER TABLE tableNameHere StatusOrYourColumn INT AUTO_INCREMENT PRIMARY KEY
But be careful, you don't want to overwrite your settings that you have already set.
Another Thing that came to my mind when reading was where you said Status and Userid where the same. You can find these using the WHERE clause like so:
SELECT * FROM tableName WHERE tableName.Userid = anotherTableOrTableName.Status
Using these queries you can update, remake, alter and query your database table.
I want a table with an integer column, that may or may not be filled (it is a social security number). But if it is filled, I want it to be UNIQUE : there cannot be two entries of the same number.
Using a unique constraint won't work cause integer won't accept NULL values, and MySQL detects multiple 0 values.
How can I set a unique constraint on an integer with a default value ? Or how can I set the integer column to accept NULL values ? (this question takes it for granted : MySQL Foreign Key Constraint - Integer Column but I can't)
create table test (
myint INT NULL, UNIQUE INDEX (myint)
);
This will allow a unique constraint on any integers added but will allow multiple NULL values to be entered.
MySQL treats NULL as 'unknown' value so cant possibly do a comparison to see if a like value is already there 'unknown' !== 'unknown'.
This also depends on which database engine you are using, the above holds true for MyISAM and InnoDB
Please see the code below:
ALTER TABLE test MODIFY myint INT NULL
ALTER TABLE test ADD UNIQUE INDEX (myint)
It works when data inputs are directly from MySQL (PHPMyadmin), saved as NULL, but a php script saves it as zeroes, and so it does not allow multiple entries.
I am entering records in the MySQL DB. Now I want to have a "Serial_Number" field that increements automatically whenever a record is entered into the DB.
I don't want this "Serial_Number" field to be the primary key of the DB.
How can I create this field (with the attributes needed to be set).
I am using "SQL YOG" to access MySQL. If you are aware of the SQL YOG then tell me how to do that through SQL YOG.
The AUTO_INCREMENT column has to have a UNIQUE KEY constraint associated to it.
For instance, this will work just fine:
CREATE TABLE AutoNotId
(
Id INTEGER PRIMARY KEY,
Auto INTEGER AUTO_INCREMENT,
UNIQUE (Auto)
);
Edit:
The ALTER statement would look somewhat like this:
ALTER TABLE AutoNotId
MODIFY COLUMN Auto INTEGER AUTO_INCREMENT,
ADD UNIQUE (Auto);
I recommended, however the use of the long-hand syntax to specify the name of the UNIQUE constraint; But you can always refer to MySQL's Reference Manual for the exact specifications.
In MySQL tables can only have one auto increment field and they must be indexed.
There can be only one AUTO_INCREMENT column per table, it must be indexed, and it cannot have a DEFAULT value.
Is there a reason you don't want it to be the primary key?
If you want an incrementing value, you could fudge it by running updates after each insert:
SELECT MAX(serial) + 1 FROM myTable;
UPDATE myTable SET serial = <that number> WHERE id = ...
I don't think you can have an auto increment field:
CREATE TABLE `t` (`dd` int(11) NOT NULL)
ALTER TABLE `t` CHANGE `dd` `dd` INT( 11 ) NOT NULL AUTO_INCREMENT
MySQL said: Documentation
#1075 - Incorrect table definition; there can be only one auto column and it must be defined as a key
You cannot do this in MySQL. From the doc:
There can be only one AUTO_INCREMENT
column per table, it must be indexed,
and it cannot have a DEFAULT value. An
AUTO_INCREMENT column works properly
only if it contains only positive
values. Inserting a negative number is
regarded as inserting a very large
positive number. This is done to avoid
precision problems when numbers “wrap”
over from positive to negative and
also to ensure that you do not
accidentally get an AUTO_INCREMENT
column that contains 0.
For MyISAM and BDB tables, you can
specify an AUTO_INCREMENT secondary
column in a multiple-column key. See
Section 3.6.9, “Using AUTO_INCREMENT”.
create table mytable (
ID INT IDENTITY(1,1) PRIMARY KEY,
SN INT IDENTITY(1,1)
)
I am using MySQL for my database. I have a requirement to store a list of users in the system. The user will have both first name and last name.
I cannot have the first name and second name as the primary key. So I need a indexing key. But I do not want to enter the index value every time to add a new row.
I want the system to handle this with auto increment of previous value every time I try to add a new user. Please let me know how to do this.
Also I am supposed to do a search operation on the list of user with the first name. Please let me know the efficient way to do the search as the number of records could be more than a million.
create table users (
id int primary key auto_increment,
name varchar(64),
...
)
See here for more info
Add an INT() id column with auto_increment set. This way, the id column value will be incremented automatically on each INSERT.
To get a unique key without having to specify it, you need an auto_increment field:
create table people (
person_id int primary key auto_increment,
first_name varchar(50),
: : :
);
For efficiently searching first names, you just need an index on the first-name column. A couple of million rows is not a big table, so this should be reasonably efficient.
create index person_index using btree on people (first_name);