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ON DUPLICATE KEY UPDATE syntax [5] which can be used to achieve a similar effect with the limitation that the join between target and source has to be made only on PRIMARY KEY or UNIQUE constraints, which is not required in the ANSI/ISO standard.
Names for footnotes and groups must follow these rules: Names are case-sensitive. Please do not use raNdOM capitalization. Names must not be purely numeric; the software will accept something like ":31337" (which is punctuation plus a number), but it will ignore "31337" (purely numeric).
For referential integrity to hold in a relational database, any column in a base table that is declared a foreign key can only contain either null values or values from a parent table's primary key or a candidate key. [2] In other words, when a foreign key value is used it must reference a valid, existing primary key in the parent table.
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In SQL, the unique keys have a UNIQUE constraint assigned to them in order to prevent duplicates (a duplicate entry is not valid in a unique column). Alternate keys may be used like the primary key when doing a single-table select or when filtering in a where clause, but are not typically used to join multiple tables.
A "do not duplicate" key (or DND key, for short) is one that has been stamped "do not duplicate", "duplication prohibited" or similar by a locksmith or manufacturer as a passive deterrent to discourage a retail key cutting service from duplicating a key without authorization or without contacting the locksmith or manufacturer who originally cut ...