Postgresql – Constrain input to a few different strings

constraintpostgresql

Hi I can't seem to get a constraint working the way I expect in postgreSQL.
From within pgadmin I execute the following SQL query.

-- Check: "TypeCheck"

-- ALTER TABLE "ComLog" DROP CONSTRAINT "TypeCheck";

ALTER TABLE "ComLog"
  ADD CONSTRAINT "TypeCheck" CHECK ("Type" SIMILAR TO 'email|post|IRL|minutes');
COMMENT ON CONSTRAINT "TypeCheck" ON "ComLog" IS 'One of email|post|IRL|minutes';

When executed this is converted to.

-- Check: "TypeCheck"

-- ALTER TABLE "ComLog" DROP CONSTRAINT "TypeCheck";

ALTER TABLE "ComLog"
  ADD CONSTRAINT "TypeCheck" CHECK ("Type" ~ similar_escape('email|post|IRL|minutes'::text, NULL::text));
COMMENT ON CONSTRAINT "TypeCheck" ON "ComLog" IS 'One of email|post|IRL|minutes';

I expect this to limit my input for the Types column to one of email post IRL or minutes. However when entering table data this constraint fails when i enter one of these types.
The Types column is of character type.
Does anyone know how to fix this.
Thanks.

Best Answer

Change your constraint to

CHECK (type IN ('email','post','IRL','minutes'))

This will be converted by the parser into:

CHECK (type = ANY( ARRAY['email','post','IRL','minutes']))

That should do what you are looking at.

However I have to wonder if it wouldn't be better to do this:

CREATE TABLE comlog_types (
     type text
);

And then add a foreign key to enforce the constraint. This would make it easier to add types in the future.

Related Solutions

Sql-server – Check constraint only one of three columns is non-null

The following should do the trick:

CREATE TABLE MyTable (col1 FLOAT NULL, col2 NVARCHAR(30) NULL, col3 DATETIME NULL);
GO

ALTER TABLE MyTable
ADD CONSTRAINT CheckOnlyOneColumnIsNull
CHECK 
(
    ( CASE WHEN col1 IS NULL THEN 0 ELSE 1 END
    + CASE WHEN col2 IS NULL THEN 0 ELSE 1 END
    + CASE WHEN col3 IS NULL THEN 0 ELSE 1 END
    ) = 1
)
GO

Sql-server – Altering nullability of column in SQL Server

The relevant documentation is here

ALTER COLUMN Specifies that the named column is to be changed or altered.

The modified column cannot be any one of the following:

Used in an index, unless the column is a varchar, nvarchar, or varbinary data type, the data type is not changed, the new size is equal to or larger than the old size, and the index is not the result of a PRIMARY KEY constraint.

Used in statistics generated by the CREATE STATISTICS statement unless the column is a varchar, nvarchar, or varbinary data type, the data type is not changed, and the new size is equal to or greater than the old size, or if the column is changed from not null to null. First, remove the statistics using the DROP STATISTICS statement. Statistics that are automatically generated by the query optimizer are automatically dropped by ALTER COLUMN.

In practice it seems that SQL Server does allow some additional cases beyond that mentioned in the documentation however.

As you show in your question ALTER TABLE test ALTER COLUMN field1 int null does in fact work so the restrictions on changes to columns used in indexes appear to be the same as those for user created statistics.

Moreover the caveat mentioned about Primary Keys seems to be untrue also. The following works fine.

CREATE TABLE test2(pk varchar(10) primary key);

ALTER TABLE test2 ALTER COLUMN pk varchar(100) NOT NULL

Best Answer

Related Solutions

Sql-server – Check constraint only one of three columns is non-null

Sql-server – Altering nullability of column in SQL Server

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