1. Foreign Key
2. Primary Key
3. Generate Unique Key In Php Code

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Note that MySQL will not add a unique constraint if the existing data in the columns of specified in the unique constraint does not comply with the uniqueness rule. In this tutorial, you have learned how to use the MySQL UNIQUE constraint to enforce the uniqueness of values in a.

1. One can apply arraykeys twice to get the position of an element from its key. (This is the reverse of the function by cristianDOTzuddas.) E.g., the following may output 'yes, we have bananas at position 0'.
2. I would like to create a unique key for both account activation and referral purposes, that includes a checksum to help prevent users from easily guessing other users activation or referral keys. Also, in PHP is it possible to create you own session.

APPLIES TO: SQL Server 2016 and later Azure SQL Database Azure Synapse Analytics (SQL DW) Parallel Data Warehouse

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

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• Before you begin:

• To create a unique constraint, using:

Before You Begin

Security

Permissions

Foreign Key

Requires ALTER permission on the table.

Using SQL Server Management Studio

To create a unique constraint

1. In Object Explorer, right-click the table to which you want to add a unique constraint, and click Design.

2. On the Table Designer menu, click Indexes/Keys.

3. In the Indexes/Keys dialog box, click Add.

4. In the grid under General, click Type and choose Unique Key from the drop-down list box to the right of the property.

5. On the File menu, click Savetable name.

Using Transact-SQL

To create a unique constraint

1. In Object Explorer, connect to an instance of Database Engine.

2. On the Standard bar, click New Query.

3. 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 column TransactionID.

Primary Key

To create a unique constraint on an existing table

1. In Object Explorer, connect to an instance of Database Engine.

2. On the Standard bar, click New Query.

3. Copy and paste the following example into the query window and click Execute. The example creates a unique constraint on the columns PasswordHash and PasswordSalt in the table Person.Password.

To create a unique constraint in an new table

Generate Unique Key In Php Code

1. In Object Explorer, connect to an instance of Database Engine.

2. On the Standard bar, click New Query.

3. 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).