How do I create a unique constraint on a table?
I tried doint this but it chokes on UPPER.
ALTER TABLE MY_TABLE ADD UNIQUE(UPPER(MY_COL))
Thanks,
If you want to use a function you would need to use the CREATE INDEX statement. The keyword "unique" makes it a unique constraint.
CREATE UNIQUE INDEX MyUpperIndex ON MY_TABLE(UPPER(MY_COL))
Related
I am importing data from an access table to SQL. I have a primary key is SQL "quoteID" which obviously doesn't allow duplicates but I'm looking to add that requirement to another field.
Can't seem to find where to set that? perhaps it has to do with the field type??
You can require that a column be unique using a unique constraint or unique index:
alter table t add constraint unq_t_col unique (col);
or:
create unique index unq_t_col on t(col);
Make the field a primary key of the table. Or put an index on that field that enforces unique values.
SQLite doesn't support Add Constaints. I have the below SQL. How could I get that supported?
ALTER TABLE mytable ADD CONSTRAINT unique_hash UNIQUE (hash) ON CONFLICT REPLACE
Note: CREATE UNIQUE INDEX won't solve this problem of having ON CONFLICT REPLACE
How do prevent a table with 2 FOREIGN key's from having repeated rows with the same value.
Thks in advance.
Use a unique index or constraint:
alter table example
add constraint unq_example_fk1_fk2 unique (fk1, fk2);
A unique constraint and unique index do essentially the same thing. So, you can also do:
create unique index unq_example_fk_fk on example(fk1, fk2);
I have a table in Oracle 11g such as with below columns.
COL1_STATUS
COL2_ID
COL3_TYPE
COL4_DATE
I want to create a UNIQUE constraint combining all 4 columns but only when COL1_STATUS = 10
How can I do that? Table is already created so I am looking for only ALTER command.
Also, I have searched and found similar question where it is suggested to use unique index but I want to achieve this by constraint.
Conditional unique constraint with multiple fields in oracle db
Thanks in advance.
A unique index and a constraint are essentially the same thing. A unique constraint is implement using a unique index. So this really should do what you want:
create unique index idx_table_4 on
table(case when status = 10 then id end,
case when status = 10 then type end,
case when status = 10 then date end);
In fact, this is how the documentation recommends implementing a unique constraint:
When you specify a unique constraint on one or more columns, Oracle
implicitly creates an index on the unique key. If you are defining
uniqueness for purposes of query performance, then Oracle recommends
that you instead create the unique index explicitly using a CREATE
UNIQUE INDEX statement. You can also use the CREATE UNIQUE INDEX
statement to create a unique function-based index that defines a
conditional unique constraint. See "Using a Function-based Index to
Define Conditional Uniqueness: Example" for more information.
In SQL Server 2005+ (I use both), does adding the UNIQUE constraint to a column automatically create an index, or should I still CREATE INDEX?
See this MSDN article:
The Database Engine automatically
creates a UNIQUE index to enforce the
uniqueness requirement of the UNIQUE
constraint.
If you do create an index, you'll end up with two indexes, as this example demonstrates:
create table TestTable (id int)
alter table TestTable add constraint unique_id unique (id)
create unique index ix_TestTable_id on TestTable (id)
select * from sys.indexes where [object_id] = object_id('TestTable')
This will display two unique indexes on TestTable; and the HEAP that represents the table itself.
Yes, it does.
In fact, you can even create a CLUSTERED UNIQUE CONSTRAINT:
ALTER TABLE mytable ADD CONSTRAINT UX_mytable_col1 UNIQUE CLUSTERED (col1)
, which will make the table to be clustered on col1.
Almost all databases create an index for UNIQUE CONSTRAINT, otherwise it would be very hard to maintain it.
Oracle doesn't even distinguish between UNIQUE CONSTRAINT and UNIQUE INDEX: one command is just a synonym for another.
The only difference in Oracle is that a UNIQUE INDEX should have a user-supplied name, while a UNIQUE CONSTRAINT may be created with a system-generated name:
ALTER TABLE mytable MODIFY col1 UNIQUE
This will create an index called SYS_CXXXXXX.
An index is created when you add a unique constraint:
Reference -- see the second paragraph.
When a UNIQUE constraint is added to
an existing column or columns in the
table, by default, the Database Engine
examines the existing data in the
columns to make sure all values are
unique. If a UNIQUE constraint is
added to a column that has duplicated
values, the Database Engine returns an
error and does not add the constraint.
The Database Engine automatically
creates a UNIQUE index to enforce the
uniqueness requirement of the UNIQUE
constraint. Therefore, if an attempt
to insert a duplicate row is made, the
Database Engine returns an error
message that states the UNIQUE
constraint has been violated and does
not add the row to the table. Unless a
clustered index is explicitly
specified, a unique, nonclustered
index is created by default to enforce
the UNIQUE constraint.