See we have one table with unique key on one field,
now whenever we are inserting any record,how mysql checking that that field value is exist in table or not?
whats the logic here?
its internally fires queries to check that that value is exist in table ? I think thats not possible.
here unique filed can be any id,email,username anything
MySQL creates index for primary and unique keys. So it checks index to see that new inserting value is unique actually. Since it is checking index, it does not hurt performance.
A UNIQUE index creates a constraint such that all values in the index
must be distinct. An error occurs if you try to add a new row with a
key value that matches an existing row. For all engines, a UNIQUE
index permits multiple NULL values for columns that can contain NULL.
Well it has to find the record in the index anyway when you inserting a value in order to write the data and to update all the indexes with your new value. Not much trouble to read that location and compare it to the new values.
so yes it just runs your query.
Related
I have a field that stores product codes. The codes are unique, but some products simply doesn't have a code. I can't invent codes because those are providers codes.
Is this kind of constraint possible in MySQL?
I'm a noob with stored procedures and triggers, so if the solution involves one of these, please be patient.
Update:
The column is NOT Null. That's why I was unable to do this.
Yes, you can do this. See the MySQL reference (version 5.5).
A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. For all engines, a UNIQUE index allows multiple NULL values for columns that can contain NULL.
Yes, if you make the product code column nullable (not declared with NOT NULL), the unique key will allow multiple rows with NULL product codes.
MySQL still allows for multiple rows to have a value of NULL in a unique column.
I am trying to add a row in existing table I want to make it unique when I am trying to alter in phpmyadmin it says #1062 - Duplicate entry '0' for key 'mobile'
What code will help me suggest
This indicates that you have two entries which both have '0' in the mobile column. You can't force the column to be UNIQUE because there's non-unique data in their now. The solution is to resolve the conflict, but whether that's a good idea and how exactly to do so depends on your database design. The act of doing it is rather simple (just edit that row and assign a new value for 'mobile'), but depending on your design that could damage some data.
So without knowing the details, I can only caution you to not destroy any data or relations.
If you don't have a primary or unique key in that table, phpMyAdmin doesn't show the '"grid edit" feature, so if that's the case you can either write a little SQL to update the row directly, or temporarily add a new column, make it an autoincrementing primary key, do the edit through the phpMyAdmin interface, then remove the temporary autoincrement column (that's what I'd do; I just tested it and it took me about 30 seconds to add the column and key, edit a row, and delete the temporary column).
This has been discussed before, however I cannot understand the answers I have found.
Essentially I have a table with three columns memo, user and keyid (the last one is primary and AUTO_INC). I insert a pair of values (memo and user). But if I try to insert that same pair again it should not happen.
From what I found out, the methods to do this all depend on a unique key (which I've got, in keyid) but what I don't understand is that you still need to do a second query just to get the keyid of the existing couple (or get nothing, in which case you go ahead with the insertion).
Is there any way to do all of this in a single query? Or am I understanding what I've read (using REPLACE or IGNORE) wrong?
You need to set a UNIQUE KEY on user + memo,
ALTER TABLE mytable
ADD CONSTRAINT unique_user_memo UNIQUE (memo,user)
and then using INSERT IGNORE or REPLACE according to your needs when inserting. Your current unique key is the primary key, that is all well and good, but you need a 2nd one in order to not allow the insertion of duplicate data. If you do not create a new unique key on the two columns together, then you'll need to do a SELECT query before every insert to check if the pair already exists.
I have a field that stores product codes. The codes are unique, but some products simply doesn't have a code. I can't invent codes because those are providers codes.
Is this kind of constraint possible in MySQL?
I'm a noob with stored procedures and triggers, so if the solution involves one of these, please be patient.
Update:
The column is NOT Null. That's why I was unable to do this.
Yes, you can do this. See the MySQL reference (version 5.5).
A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. For all engines, a UNIQUE index allows multiple NULL values for columns that can contain NULL.
Yes, if you make the product code column nullable (not declared with NOT NULL), the unique key will allow multiple rows with NULL product codes.
MySQL still allows for multiple rows to have a value of NULL in a unique column.
In postgres we have a constraint defined that essentially allows us to limit the number of entries in a table with a certain value to one. We created this constraint:
create unique index list$default$uk on list_group(visitor_uid) where list_type = 'default';
Which means that the unique constraint is only applied when the list_type='default' so that there can only be one 'default' list per visitor in the table.
It looks like MySql doesn't support the additional where on the unique constraint. Is there another way to support this in the database schema for MySQL?
MYSQL doesn't support such types of constraints.
You should use stored procudures for inserting data instead, so you can do some checking and validation.
Why don't you define your default as such that it must have 1 as primary key? This way a normal unique constraint on the pk would be enough already.
If nothing fits to you, you could also consider changing your data model.
Actually it exists. Most contraints depend on the table Engine.
I think InnoDB supports this.
To do it, you have to add a UNIQUE index with the unique combination:
ALTER TABLE visitor ADD UNIQUE unique_default_visitor( visitor_uid, list_type );
I supposed you'd have to write a trigger to check for it.
I never use MySQL but maybe you can create an index like this:
CREATE UNIQUE INDEX list$default$uk ON list_group ((CASE WHEN list_type='default' THEN NULL ELSE visitor_uid END));
Explanation:
A unique index should not care about NULL values. Therefore, make sure the index expression returns NULL for every row where list_type <> 'default'.
My two cents:
why don't you create a column only to store the unique value (maybe you can call it something like is_list_type_default) and another to store all the values. If you do this, you can put a unique constraint on the first column.