When designing an SQL database, there may be cases where you want to impose restrictions on what data can be added to certain columns in a table. SQL makes this possible through the use ofconstraints. After applying a constraint to a column or table, any attempts to add data to the colu...
TheOrderIdandProductIdcolumns uniquely identify each row in theOrderProducttable. To give it a physical meaning, each product can be present in many orders; hence, theProductIdcolumn can store duplicate values. Also, each order can have many products; theOrderIdcolumn can also store duplicate v...
Here we added unique constraints on a set of columns. We can also add multiple unique constraints, meaning unique constraints on multiple sets of columns. We’ll do just that in the next section. 5.3. Multiple Unique Constraints on a Single Entity A table can have multiple unique constraints....
The primary key is a column that is normally referenced in queries, particularly to join tables. The column EMP_PHONE has been designated as a UNIQUE value, meaning that no two employees can have the same telephone number. There is not a lot of difference between the two, except that the...
In case your model defines a range using two fields, instead of the native PostgreSQL range types, you should write an expression that uses the equivalent function (e.g.TsTzRange()), and use the delimiters for the field. Most often, the delimiters will be'[)', meaning that the lower bo...
1 SQL> delete from daddy where id = 112;The delete hangs in a locked state due to the child transaction in Session 1. This can lead to further locking issues in other sessions (Session 3):1 insert into kiddy values (10000002,1042,1042,1042,'Iggy Pop');...
In previous schemes, multi-table constraints, at least (and possibly others too), were checked atCOMMITtime (that is, at end-of-transaction) instead of immediately. In particular, the SQL standard includes something it calls "DEFERRED checking" (though in SQL the checking is done atCOMMITtime...
Anyway, you can see that we've used the POSSREP in the example to specify a type constraint (see the WHERE clause): Quantities must be such that they can be represented by, precisely, the integers 1, 2, ..., 5000 -- meaning (loosely) that legal quantities are just those five thousa...
Check constraint • Not Null constraint, which is really a special case of a Check constraint 3.1 Primary Key The PRIMARY KEY of a table constrains a single column, or set of columns, to a unique and mandatory value—mandatory, meaning that no column in the primary key can ever be null...
TiDB supports foreign key constraint check, which is controlled by the system variableforeign_key_checks. By default, this variable is set toON, meaning that the foreign key constraint check is enabled. This variable has two scopes:GLOBALandSESSION. Keeping this variable enabled can ensure the int...