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If it does, then only one of the join rows will be used to update the target row, but which one will be used is not readily predictable. [2] Because of this indeterminacy, referencing other tables only within sub-selects is safer, though often harder to read and slower than using a join. MySQL does not conform to ANSI standard. [3]
In relational databases, the log trigger or history trigger is a mechanism for automatic recording of information about changes inserting or/and updating or/and deleting rows in a database table. It is a particular technique for change data capturing , and in data warehousing for dealing with slowly changing dimensions .
Row level triggers would execute once for each row that is affected by the UPDATE. It is important to keep in mind if no rows are affected by the UPDATE command, the trigger will not execute any code within the trigger. Statement level triggers will be called once regardless of how many rows are affected by the UPDATE. Here it is important to ...
There is also an IGNORE clause for the INSERT statement, [7] which tells the server to ignore "duplicate key" errors and go on (existing rows will not be inserted or updated, but all new rows will be inserted). SQLite's INSERT OR REPLACE INTO works similarly. It also supports REPLACE INTO as an alias for compatibility with MySQL. [8]
Inserts are easy too because new rows are appended to the end of the data file. However, delete and update operations are more problematic: deletes must leave an empty space, or the rows' offsets would change; the same goes for updates, as the length of the rows becomes shorter; if the update makes the row longer, the row is fragmented.
The WHERE clause eliminates all rows from the result set where the comparison predicate does not evaluate to True. The GROUP BY clause projects rows having common values into a smaller set of rows. [clarification needed] GROUP BY is often used in conjunction with SQL aggregation functions or to eliminate duplicate rows from a result set.
Cursors can not only be used to fetch data from the DBMS into an application but also to identify a row in a table to be updated or deleted. The SQL:2003 standard defines positioned update and positioned delete SQL statements for that purpose. Such statements do not use a regular WHERE clause with predicates. Instead, a cursor identifies the row.
For optimistic locking each row has an independent version number, typically a sequential counter. This allows a process to atomically update a row and increment its counter only if another process has not incremented the counter. But CDC cannot use row-level versions to find all changes unless it knows the original "starting" version of every row.