mysql unique key multiple null

Yes, MySQL allows multiple NULLs in a column with a unique constraint. CREA...

Yes, MySQL allows multiple NULLs in a column with a unique constraint. CREATE TABLE table1 (x INT NULL UNIQUE); INSERT table1.

⬇ Download Full Version

Thus they don't violate the unique key constraint. make working with u...

Thus they don't violate the unique key constraint. make working with unique indexes and nullable fields almost impossible, for NULL could as.

⬇ Download Full Version

If you use a UNIQUE constraint but don't want multiple rows with NULL,...

If you use a UNIQUE constraint but don't want multiple rows with NULL, declare the columns as NOT NULL and prohibit any row from having.

⬇ Download Full Version

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.

⬇ Download Full Version

The SQL standard says: "A unique constraint is satisfied if and only i...

The SQL standard says: "A unique constraint is satisfied if and only if no two rows in a table have the same non-null values in the unique.

⬇ Download Full Version

This constraint does not apply to NULL values except for the BDB storage en...

This constraint does not apply to NULL values except for the BDB storage engine. For other engines, a UNIQUE index permits multiple NULL.

⬇ Download Full Version

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.

⬇ Download Full Version

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.

⬇ Download Full Version

The constraint is skipped / accepted when at least one value is null. MySQL...

The constraint is skipped / accepted when at least one value is null. MySQL documentation on CREATE TABLE is not very clear but you can.

⬇ Download Full Version

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.

⬇ Download Full Version

When you create a unique constraint, MySQL creates a UNIQUE index behind th...

When you create a unique constraint, MySQL creates a UNIQUE index behind the Therefore, you can have multiple NULL values in the UNIQUE index.

⬇ Download Full Version

As documented in the MySQL Reference Manual: a UNIQUE index permits multipl...

As documented in the MySQL Reference Manual: a UNIQUE index permits multiple NULL values for columns that can contain NULL. There are many scenarios in which this behavior Field | Type | Null | Key | Default | Extra.

⬇ Download Full Version

As you know, when you create a UNIQUE constraint on a nullable column, uniq...

As you know, when you create a UNIQUE constraint on a nullable column, uniqueness on a column and also allow multiple NULL values.

⬇ Download Full Version

This post covers unique indexes in MySQL int unsigned not null, -- number o...

This post covers unique indexes in MySQL int unsigned not null, -- number of hours since unix epoch impressions int unsigned not null default 0, primary key(hour)); You can insert multiple rows in a single INSERT .

⬇ Download Full Version

Although both a UNIQUE constraint and a PRIMARY KEY constraint Unlike PRIMA...

Although both a UNIQUE constraint and a PRIMARY KEY constraint Unlike PRIMARY KEY constraints, UNIQUE constraints allow for the value NULL. You can also apply a single CHECK constraint to multiple columns by.

⬇ Download Full Version