unique key mysql null values

A fundamental property of a unique key is that it must be unique. treat NUL...

A fundamental property of a unique key is that it must be unique. treat NULL as a value that can only be inserted once into a UNIQUE column.

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A UNIQUE Constraint makes it impossible to COMMIT any operation that would ...

A UNIQUE Constraint makes it impossible to COMMIT any operation that would cause the unique key to contain any non-null duplicate values.

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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.

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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 CREATE TABLE t1 (col1 INT NOT NULL, col2 DATE NOT NULL, col3 INT NOT . PARTITION p2 VALUES LESS THAN (30), -> PARTITION p3 VALUES LESS.

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Hello, all! I have the following table in Oracle: SQL> create table myta...

Hello, all! I have the following table in Oracle: SQL> create table mytable(name varchar2(10) not null, ref number(9,0), constraint uk unique.

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use the BDB-Storage engine in MySQL prior which treats nulls as I will make...

use the BDB-Storage engine in MySQL prior which treats nulls as I will make my NULL column a NOT NULL column with default value 0.

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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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for MySQL I saw frequent complaints that the UNIQUE constraint didn't ...

for MySQL I saw frequent complaints that the UNIQUE constraint didn't INTO t VALUES (NULL); /* This does not cause a "unique constraint.

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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.

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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.

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Regardless of using unique constraint or unique index, the field can accept...

Regardless of using unique constraint or unique index, the field can accept null values, however the uniqueness will result in only accepting a.

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The UNIQUE constraint ensures that all values in a column are different. My...

The UNIQUE constraint ensures that all values in a column are different. MySQL: CREATE TABLE Persons (ID int NOT NULL, LastName varchar() NOT.

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Column c2 is part of a primary key, and thus it cannot be NULL. 5) If the u...

Column c2 is part of a primary key, and thus it cannot be NULL. 5) If the unique specification> specifies PRIMARY KEY, then for each in MariaDB and MySQL instead mark such a column as “not having a default value”.

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UNIQUE (VALUE) [ ] UNIQUE that would cause the unique key to contain any no...

UNIQUE (VALUE) [ ] UNIQUE that would cause the unique key to contain any non-null duplicate values. (Multiple.

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

Here is an example with composite unique key and foreign key on Oracle. SQL> insert into TABLE1 values ('X',null); 1 row inserted. This is what MySQL and PostgresSQL do: accept duplicates when there is at least one.

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