The itemNumber column provides a unique identity for each item (or row) and has an attached column-level constraint that enforces a valid value to always be supplied (NOT NULL).
itemNumber列提供了每个商品（或列）的惟一标识，它上面还有列级约束，强制要求提供正确的值（NOT NULL）。
Syscat.tabconst Contains a row for each unique (u), primary key (p), foreign key (f), or table check (k) constraint db2 select constname, tabname, type from syscat.tabconst.
tabconst为每个唯一性(U)、主键(p)、外键(f)或者表检查(K)约束保持一个行db2select constname,tabname,typefrom syscat . tabconst。
SYSCAT.COLCHECKS Contains a row for each column that is referenced by a table check constraint db2 select constname, tabname, colname, usage from syscat.colchecks
COLCHECKS为表检查约束所引用的每个列保持一个行db2select constname,tabname,colname,usage from syscat.colchecks
When you attach a unique constraint to a column allowing null values, you ensure that at most one row will have a null value in the constrained column.
当将唯一约束附加到允许空值的列时，请确保在约束的列中最多有一行包含空值。
The greatest lower bound of constraint was the bound of tasks for satisfying the constraint and can be used to guarantee the "meet row" weakly hard real-time constraint.
下确界限制用于保证任务满足“至少满足”弱硬实时限制，是任务满足限制的下确界。
The greatest lower bound of constraint was the bound of tasks for satisfying the constraint and can be used to guarantee the "meet row" weakly hard real-time constraint.
下确界限制用于保证任务满足“至少满足”弱硬实时限制，是任务满足限制的下确界。
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