mysql unique null key

A fundamental property of a unique key is that it must be unique. Making pa...

A fundamental property of a unique key is that it must be unique. Making part of that key Nullable destroys this property. There are two possible.

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For MySql, the unique constraints does ignore null values but it shouldn�...

For MySql, the unique constraints does ignore null values but it shouldn't. This ensures that any key dependency on the column can be.

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Yes, this is the expected behaviour in MySQL (in fact in ANSI too). NULL va...

Yes, this is the expected behaviour in MySQL (in fact in ANSI too). NULL values are not treated as equal values in unique keys and primary.

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The idea is to add a key that deals with NULL as a concrete value, like �...

The idea is to add a key that deals with NULL as a concrete value, like '0', index the combination of the fields that you want to be unique.

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Possibilities, you can: use the BDB-Storage engine in MySQL prior which tre...

Possibilities, you can: use the BDB-Storage engine in MySQL prior which treats nulls as non-distinct to each other. Later versions don't.

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But I hope everyone knows what NULL means, the key point is the behaviour f...

But I hope everyone knows what NULL means, the key point is the behaviour for NULL being distinct (when it comes to index) has its uses.

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In other words, every unique key on the table must use every column in the ...

In other words, every unique key on the table must use every column in the table's mysql> CREATE TABLE t3 (-> col1 INT NOT NULL, -> col2 DATE NOT.

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Unlike the PRIMARY KEY index, you can have more than one UNIQUE Unlike othe...

Unlike the PRIMARY KEY index, you can have more than one UNIQUE Unlike other database systems, MySQL considers NULL values as distinct values.

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When I worked for MySQL I saw frequent complaints that the UNIQUE constrain...

When I worked for MySQL I saw frequent complaints that the UNIQUE constraint didn't stop users from inserting NULLs, multiple times.

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Here is an example with composite unique key and foreign key on Oracle. the...

Here is an example with composite unique key and foreign key on Oracle. the non-null columns of a partially null composite unique key constraint. This is what MySQL and PostgresSQL do: accept duplicates when there.

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UNIQUE implicitly requires an equality test, and NULL is not equal to NULL ...

UNIQUE implicitly requires an equality test, and NULL is not equal to NULL by rule (the Greg Kemnitz, Postgres internals, embedded device db internals, MySQL user-level. Answered Aug 5, UNIQUE implicitly requires an equality test.

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This post covers unique indexes in MySQL of hours since unix epoch impressi...

This post covers unique indexes in MySQL of hours since unix epoch impressions int unsigned not null default 0, primary key(hour));.

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If you do not define a PRIMARY KEY for your table, MySQL locates the first ...

If you do not define a PRIMARY KEY for your table, MySQL locates the first UNIQUE index where all the key columns are NOT NULL and.

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MySQL allowing one null on a NOT NULL UNIQUE column: Can't use PRIMARY...

MySQL allowing one null on a NOT NULL UNIQUE column: Can't use PRIMARY KEY because there already is one.

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Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniquen...

Both the UNIQUE and PRIMARY KEY constraints provide a guarantee for uniqueness for a MySQL: CREATE TABLE Persons (ID int NOT NULL, LastName.

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