when creating a new table i'm defining a field called ID with index primary and autoincrement.
when adding new records it's not possible defining the same ID twice so i was wondering: what's the deal about setting index "unique" for ID? is it required, will it improve performance? thanks
You dont have to set it unique, by setting it as auto increment and primary key you have already forced uniqueness over there. You dont need seperate unique constraint here.
You can not add the multiple records with the same ID.
if you want to create the multiple record , firstly delete the primary or auto incr...
Then it will be inserted.
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In MySQL, I understand below
AUTO_INCREMENT: used to make INT value to increase automatically every time a user creates a row.
PRIMARY KEY: used to make value unique in that table.
However, I oftentimes see them used together when defining id.
I don't understand the point of using AUTO_INCREMENT with PRIMARY KEY since AUTO_INCREMENT itself would make id unique.
Is there any reason they are used together when creating id?
The primary key has three properties:
It is unique.
It is non-null.
There is only one per table.
Defining the key as a primary key means that it should also be used for foreign key references.
In addition, MySQL clusters the data by the primary key. So the declaration instructs new rows to go at the "end" of the table -- meaning adjacent to the most recent inserts on the data pages.
In addition, duplicate values for the auto-incremented id could be created in various ways. One way is that the increment counter can be reset, causing duplicates. MySQL should be pretty thread-safe on duplicates for concurrent updates, but bugs have been reported. As a primary key, no duplicates will be allowed into the table.
You understand it correctly, but they are doing different things.
PRIMARY KEY with AUTO_INCREMENT means we want this column isn't duplicate on the value and it will be auto increase if we didn't set the value.
but how about we only set AUTO_INCREMENT it only means it will be auto increase if we didn't set the value. but didn't make sure the value is unique.
AUTO_INCREMENT doesn't make the column uniqe. It only "automatically" fills a value when creating a row, if missing. But you can later update the values, or also create a row by explicitly providing the value.
PRIMARY KEY denies any modification sql statement that would cause 2 different entries storing equal values. So it guarantees you, that your DB is in a correct state.
We have mysql database with auto increment column(id) as primary key for all tables and which is referred in another table as foreign key. We have planned to use index for some of the columns in order to increase the accessing time of the table.
Is it necessary to create index for auto increment column(id) which is referred in another table as foreign key, or by default mysql using index mechanism internally for the column that is referred as foreign key. In any case, If we use index on foreign key, whether it will faster the accessing time ?
Thanks in advance.....
The primary key is used as a unique index, therefor you do not need to add an extra one.
If you want to be sure, you can always add the keywords EXPLAIN in front of one of your query to show the execution plan. It will display "use index" in the last column.
We have a giant table in an Access database with over 500k records and no PK. Is it possible to insert an auto-incrementing primary key column into an already existing Access table?
Yes, it is and can be done quite simply by editing the table and adding an autoincrement type field. The only rule is that you can only have one autoincrement per table.
I have a table in MySQL with 30 columns and thousands of entries.
Is there a way I could make every row unique, that means if a row already exists, I should not be able to enter that row again.
I can't use primary or unique keys here, because individually every column might be repeated.
I want the row to be unique.
for example:-There are is a table with
columns->name,age,height,weight.
In this column I can't make any one or two columns unique but I should not have two entries with all the same data.
You can make a unique index that includes all of the columns in your table
ALTER TABLE buyers ADD UNIQUE idx_row_unique(first_name,last_name,...);
This way you can keep a unique AUTO INCREMENT primary key for join purposes, while still ensuring that all of the data in your table is unique.
You need a composite primary key.
A composite primary key tells MySQL that you want your primary key to be a combination of fields.
More info here:
Why use multiple columns as primary keys (composite primary key)
You may create UNIQUE key on all the columns, not individual unique keys on each column. This means that the combination of the values will be unique - exactly what you need. But please note, that if any column allows null value, if the column contains null value, it will be counted as unique, even if another row contains the same values, with null for the same value.
You can make a unique index on more than one column. Just put all the columns in the index. If the columns are large you may run into issues with the maximum length of an index, but try it first and see.
You can hash the data and set the hash value as your PK, this will ensure that the rows are unique.
It is my understanding that when I make a table without a primary key that MySQL creates a sort of underlying primary key that it uses internally.
I am working with a table that does not have a primary key, but it would be very useful for my application if I could somehow access this value, assuming it does in fact exist and is retrievable.
So, I am wanting to know if I am correct in believing that such a value exists somewhere and also if it is possible to get that value.
Edit: just to make it clear, it would be very useful for my application for this table to have an incrementing int attribute. Unfortunately, it was not implemented that way. So, I am sort of grasping at straws to find a solution. What I am trying to do is select every nth row in the table (n changes). So, as you can see if there was this key, this would be very simple.
If a table has no primary key then there's no way of specifying a specific row within it because there is no way to uniquely identify an item. Even if you use a query that specifies a specific value for every column that still wouldn't be certain to only return a single row as without a primary key there's nothing to prevent duplicate rows.
However, a primary key is simply a unique index. If a table has a unique index on one or more of its columns and those columns don't accept NULLs then this is the primary key for the table in all but name.
If you table has no unique columns then you've got nothing to go on. You'll have to either make one column or combination of columns (for a composite key) unique, or add a column that serves as the primary key for the table. Fortunately it's relatively easy to add columns to a MySQL table, just add a primary key autoincrement column to the existing table.