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It uses a filter predicate to index a portion of rows in the table. A well-designed filtered index can improve query performance as well as reduce index maintenance and storage costs compared with full-table indexes. And here's an example combining a unique index with a filter predicate:
19 mar 2017 · With MySQL, I took advantage of the fact that you can have multiple NULL values in a unique index, so by adding a "tie breaker"(?) field, ucode, I was able to hack together a solution:
Summary: in this tutorial, you will learn how to use the MySQL UNIQUE index to prevent duplicate values in one or more columns in a table. Introduction to the MySQL UNIQUE index To enforce the uniqueness value of one or more columns, you often use the PRIMARY KEY constraint.
5 sty 2024 · SELECT query using a UNIQUE INDEX: SELECT * FROM tbl_name WHERE column_name = 'the_value'; INSERT query with a UNIQUE INDEX: INSERT INTO tbl_name(column1, column2, ...) VALUES ('value1', 'value2', ...); UPDATE query with a UNIQUE INDEX: UPDATE tbl_name SET column_name = 'new_value' WHERE unique_column = 'the_value'; DELETE query with a UNIQUE ...
Multiple-table syntax: UPDATE [LOW_PRIORITY] [IGNORE] table_references . SET assignment_list . [WHERE where_condition] For the single-table syntax, the UPDATE statement updates columns of existing rows in the named table with new values. The SET clause indicates which columns to modify and the values they should be given.
26 sty 2024 · ‘ON DUPLICATE KEY UPDATE’ allows you to update certain values when a UNIQUE constraint is triggered, ensuring data stays fresh without errors. Indexing and UNIQUE Constraints. MySQL automatically creates an index for each UNIQUE constraint to speed up lookups on these columns.
MySQL supports an IGNORE keyword for INSERT, UPDATE, and so forth. If you use it, MySQL ignores primary-key or unique-key violations and continues processing with the next row. See the section for the statement that you are using (Section 15.2.7, “INSERT Statement”, Section 15.2.17, “UPDATE Statement”, and so forth).
