-->APPLIES TO: SQL Server 2016 and later Azure SQL Database Azure Synapse Analytics (SQL DW) Parallel Data Warehouse
- Generate Random Unique Key Mysql
- Generate Unique Key Mysql
- Create Unique Key In Mysql
- Mysql Unique Key Index
Jul 05, 2011 All 'random' methods can lead to performance issues due to jumping around in the table. This probably won't be an issue until you have millions of users.
You can create a unique constraint in SQL Server 2019 (15.x) by using SQL Server Management Studio or Transact-SQL to ensure no duplicate values are entered in specific columns that do not participate in a primary key. Creating a unique constraint automatically creates a corresponding unique index.
Note
See Primary key, foreign key, and unique key in Azure Synapse Analytics for information on unique constraints in Azure Synapse Analytics.
In This Topic
- Before you begin:
- To create a unique constraint, using:
- Jul 13, 2016 An unique key is a key that can be a column or a group of column which defines uniqueness for that record/row in the table. In this snippet you.
- Sep 25, 2009 Hi All, In my project, I have to generate unique keys of fixed lenght(say 20 digits, alpha-numeric). How could we do this in MS Sql Server 2005 most efficiently?
Before You Begin
Security
Permissions
Requires ALTER permission on the table.
Using SQL Server Management Studio
Generate Random Unique Key Mysql
To create a unique constraint
- In Object Explorer, right-click the table to which you want to add a unique constraint, and click Design.
- On the Table Designer menu, click Indexes/Keys.
- In the Indexes/Keys dialog box, click Add.
- In the grid under General, click Type and choose Unique Key from the drop-down list box to the right of the property.
- On the File menu, click Savetable name.
Using Transact-SQL
To create a unique constraint
- In Object Explorer, connect to an instance of Database Engine.
- On the Standard bar, click New Query.
- Copy and paste the following example into the query window and click Execute. The example creates the table
TransactionHistoryArchive4
and creates a unique constraint on the columnTransactionID
.
To create a unique constraint on an existing table
- In Object Explorer, connect to an instance of Database Engine.
- On the Standard bar, click New Query.
- Copy and paste the following example into the query window and click Execute. The example creates a unique constraint on the columns
PasswordHash
andPasswordSalt
in the tablePerson.Password
.
To create a unique constraint in an new table
Generate Unique Key Mysql
- In Object Explorer, connect to an instance of Database Engine.
- On the Standard bar, click New Query.
- Copy and paste the following example into the query window and click Execute. The example creates a table and defines a unique constraint on the column
TransactionID
.For more information, see ALTER TABLE (Transact-SQL), CREATE TABLE (Transact-SQL), and table_constraint (Transact-SQL).
23.6.1 Partitioning Keys, Primary Keys, and Unique Keys
This section discusses the relationship of partitioning keys with primary keys and unique keys. The rule governing this relationship can be expressed as follows: All columns used in the partitioning expression for a partitioned table must be part of every unique key that the table may have.
In other words, every unique key on the table must use every column in the table's partitioning expression. (This also includes the table's primary key, since it is by definition a unique key. This particular case is discussed later in this section.) For example, each of the following table creation statements is invalid:
In each case, the proposed table would have at least one unique key that does not include all columns used in the partitioning expression.
Each of the following statements is valid, and represents one way in which the corresponding invalid table creation statement could be made to work:
Create Unique Key In Mysql
This example shows the error produced in such cases:
The
CREATE TABLE
statement fails because both col1
and col3
are included in the proposed partitioning key, but neither of these columns is part of both of unique keys on the table. This shows one possible fix for the invalid table definition: In this case, the proposed partitioning key
col3
is part of both unique keys, and the table creation statement succeeds. The following table cannot be partitioned at all, because there is no way to include in a partitioning key any columns that belong to both unique keys:
Since every primary key is by definition a unique key, this restriction also includes the table's primary key, if it has one. For example, the next two statements are invalid:
In both cases, the primary key does not include all columns referenced in the partitioning expression. However, both of the next two statements are valid:
Mysql Unique Key Index
If a table has no unique keys—this includes having no primary key—then this restriction does not apply, and you may use any column or columns in the partitioning expression as long as the column type is compatible with the partitioning type.
For the same reason, you cannot later add a unique key to a partitioned table unless the key includes all columns used by the table's partitioning expression. Consider the partitioned table created as shown here:
It is possible to add a primary key to
t_no_pk
using either of these ALTER TABLE
statements: However, the next statement fails, because
c1
is part of the partitioning key, but is not part of the proposed primary key: Since
t_no_pk
has only c1
in its partitioning expression, attempting to adding a unique key on c2
alone fails. However, you can add a unique key that uses both c1
and c2
. These rules also apply to existing nonpartitioned tables that you wish to partition using
ALTER TABLE ... PARTITION BY
. Consider a table np_pk
created as shown here: The following
ALTER TABLE
statement fails with an error, because the added
column is not part of any unique key in the table: However, this statement using the
id
column for the partitioning column is valid, as shown here: In the case of
np_pk
, the only column that may be used as part of a partitioning expression is id
; if you wish to partition this table using any other column or columns in the partitioning expression, you must first modify the table, either by adding the desired column or columns to the primary key, or by dropping the primary key altogether.