There may also be two or more columns which, while not unique of themselves, form a unique combination. In a database table, every row should be unique, and you need to be able to identify a particular row uniquely.Ī given table may have one or more columns which have unique values, so any of these columns can do the job. Still, I suppose that, unlike auto_increment/serial columns in MySQL/PostgreSQL, an ANSI-SQL-standard generated always as identity column doesn't allow its values to be set manually in INSERT or UPDATE ( only automatically). I mean that it makes column values to be automatically initialized with an integer sequence, but doesn't guarantee uniqueness and strict consequentiality. Microsoft SQL Server), but also a name defined by ANSI SQL standard.ĪFAIK, it doesn't differ very much from what I described above (about the common auto_increment/serial/identity feature in implementations). Update: As a_horse_with_no_name suggested, "identity" appears to be not only a name of the common auto_increment/serial/identity feature within specific RDBMSes (e.g. uniqueness (use unique/primary keys to enforce that).(usually an increasing integer sequence)Īuto_increment/serial/identity usually doesn't guarantee strict consequentiality of automatic values (especially in case of aborted transactions).Ĭoncretely documentation for TRANSACT-SQL says that identity doesn't guarantee: it'll be initialized automatically to some value not specifying any value for my_column manually, if column my_column of my_table is marked as auto_increment/serial/identity, you can do this: INSERT INTO my_table (other_column, third_column) VALUES (…, …) It's just a feature that during an row-insert operation a specific column, when not being explicitly set to some value, is automatically initialized to (most often) consecutive integer values.Į.g. Identity in your RDBMS is what other RDBMSes may call auto_increment or serial. VALUES (10, …, …) - the same value for my_column again INSERT INTO my_table (my_column, other_column, third_column) if column my_column of my_table is marked as primary or unique key, you can't do this: INSERT INTO my_table (my_column, other_column, third_column) Primary key is a bit stricter than usual unique key in that it usually implies NOT NULL and, in additional to that, only one primary key is allowed per table (while several unique keys per table are allowed in addition to primary key).Ĭreating primary/unique key usually implicitly creates an index to make search and constraint-checking by that column(s) faster.Į.g. It's, in fact, isn't required to be integer at all - it can be text or other type. Primary/unique key isn't required to be an auto-incremented. It's, in fact, a restriction (constraint) that values for a specific column (or, in general case, set of columns) cannot be the same in different rows (even when manually/explicitly setting to same values with an insert/update). Look for the same symbol, I'm sure we'll see it again.Primary key is a kind of unique key. So, soldier, I'll look for them in the woods, you do the same in the city. But with what purpose do people become these. But at least I understand the motivation there, the beast has defeated the human in their soul. Murderers who just want to shoot innocents, too. We've fought looters with only money on their minds. I keep thinking it's the worst we'll ever see, but every time I'm proven wrong. To whom? Why? So many questions, yet no answers. What the hell is going on? People are being sacrificed. It's the same story again, a new recording, now with a strange symbol.
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