Unique Key in SQL. A unique key is a set of one or more than one fields/columns of a table that uniquely identify a record in a database table. You can say that it is little like primary key but it can accept only one null value and it cannot have duplicate values. Personally, I would not go to Rome to order the sushi. However, there is the function uuid which can be used ie. SELECT uuid; and produces a guaranteed unique 36 character sitrng, but this might not be very efficient in joins as your dataset grows. Aug 09, 2008 Generate unique primary key automatically in MySQL. How to generate unique primary key automatically for every record in MySQL? Note: This question was asked by one anonymous visitor as a comment. Sharing the response as a separate article to make it more visible to our visitors. Dec 24, 2016 If you are using surrogate keys, every row is going to need to have a unique value for this column. The easiest way to do this is to just count upwards. The downside is that every time you insert.
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:
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.
Gametrials4u the sims 4 key generator reviews. 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:
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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:
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:
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However, this statement using the id column for the partitioning column is valid, as shown here:
Foreign Key
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.