This leads to a gap where the sequence is non-consecutive. In this example, the AUTO_INCREMENT value of 3 is allocated for the INSERT of the key A in INSERT INTO t (a) VALUES ('A'), ('C') ON DUPLICATE KEY UPDATE cnt = cnt + 1 but never used because this INSERT statement contains a duplicate key A. Consider the following example where consecutive AUTO_INCREMENT values of 1-3 are generated: TiDB guarantees that AUTO_INCREMENT values are monotonic (always increasing) on a per-server basis. At this time, the data whose ID is 2 already exists in the database, so the Duplicated Error error is returned. At present, because A caches the IDs of, it might assign 2 as the value of the auto-increment ID, and increases the local counter by 1. This statement does not specify the value of id, so the ID is assigned by A. The client sends a statement INSERT INTO t (c) (1) to instance A. The client inserts a statement INSERT INTO t VALUES (2, 1) to instance B, which sets id to 2. In the example above, perform the following operations in order: Otherwise, it might break the uniqueness of implicitly assigned values. With this new knowledge at our fingertips, we can rewrite our previous CREATE TABLE statement by adding our two new constraints.When the cluster has multiple TiDB instances, if the table schema contains the auto-increment IDs, it is recommended not to use explicit insert and implicit assignment at the same time, which means using the default values of the auto-increment column and the custom values. While IDENTITY can accept two arguments of the numeric seed where the values will begin from as well as the increment, these values are typically not specified with the IDENTITY constraint and instead are left as defaults (both default to 1). The second piece of the puzzle is the IDENTITY constraint, which informs SQL Server to auto increment the numeric value within the specified column anytime a new record is INSERTED. In a multi-column scenario, individual columns can contain duplicate, non-unique values, but the PRIMARY KEY constraint ensures that every combination of constrained values will in fact be unique relative to every other combination. While SQL Server only allows one PRIMARY KEY constraint assigned to a single table, that PRIMARY KEY can be defined for more than one column. The first is PRIMARY KEY, which as the name suggests, forces the specified column to behave as a completely unique index for the table, allowing for rapid searching and queries. The solution turns out to be using two constraint options provided by SQL Server. Using Identity and Primary Key Constraints When a new record is inserted, we not only must manually enter a value for id, but we have to perform a query ahead of time to attempt to verify that id value doesn’t already exist (a near-impossibility when dealing with many simultaneous connections). The problem here is, we have no way of controlling our id field. CREATE TABLE books ( id INT NOT NULL, title VARCHAR ( 100 ) NOT NULL, primary_author VARCHAR ( 100 ), )
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