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[Source] SQL creates a field unique constraint that can be empty for multiple records

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Posted on 1/5/2016 11:54:53 AM | | |

Primary key, unique constraints, and no constraints
    There are the following minor differences between primary key constraints and unique constraints.

    Primary key constraints do not allow NULL values. The index key of any index is not allowed to contain null values.However, the unique constraint allows the inclusion of NULL values, but the unique constraint treats two NULL values as duplicate values, so each column with the unique constraint on which the unique constraint is applied is allowed to contain only one NULL value.
    A clustered index is automatically created when a primary key is created, unless the current table already contains a clustered index or the NONCLUSTERED keyword is specified when the primary key is created.
    Non-clustered indexes are automatically created when a unique constraint is created, unless you specify the CLUSTERED keyword and there is no clustered index in the current table.
    There can only be one primary key in each table, but it can be constrained by multiple uniques.




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 Landlord| Posted on 6/29/2016 2:47:54 PM |
The above two sentences can realize the uniqueness of email and mobile phone numbers   

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