Foreign keys are almost always “Allow Duplicates,” which would make them unsuitable as Primary Keys. Instead, find a field that uniquely identifies each record in the table, or add a new field (either an auto-incrementing integer or a GUID) to act as the primary key. The only exception to this are tables with a one-to-one … Read more
I would use a composite (multi-column) key. This way you can have t1ID and t2ID as foreign keys pointing to their respective tables as well.
Without an index, maintaining an autoincrement column becomes too expensive, that’s why MySQL requires an autoincrement column to be a leftmost part of an index. You should remove the autoincrement property before dropping the key: Note that you have a composite PRIMARY KEY which covers all three columns and id is not guaranteed to be … Read more
A Table can have a Composite Primary Key which is a primary key made from two or more columns. For example: Update: Here is a link with a more detailed description of composite primary keys.
As the error message tells you – you can not have two rows with the same value in the columns dementia_type, snapid since they need to be unique. You have to make sure that the two databases has the same values for dementia_type, snapid. A workaround would be to add a column to your table alter table t_types_of_dementia add … Read more
Use INSERT IGNORE INTO table. There’s also INSERT … ON DUPLICATE KEY UPDATE syntax, and you can find explanations in 13.2.6.2 INSERT … ON DUPLICATE KEY UPDATE Statement. Post from bogdan.org.ua according to Google’s webcache: 18th October 2007 To start: as of the latest MySQL, syntax presented in the title is not possible. But there … Read more
Use INSERT IGNORE INTO table. There’s also INSERT … ON DUPLICATE KEY UPDATE syntax, and you can find explanations in 13.2.6.2 INSERT … ON DUPLICATE KEY UPDATE Statement. Post from bogdan.org.ua according to Google’s webcache: 18th October 2007 To start: as of the latest MySQL, syntax presented in the title is not possible. But there are several very easy ways to accomplish … Read more
Primary Key: There can only be one primary key constraint in a table In some DBMS it cannot be NULL – e.g. MySQL adds NOT NULL Primary Key is a unique key identifier of the record Unique Key: Can be more than one unique key in one table Unique key can have NULL values It can be a candidate key … Read more
Make sure you have your VendorID set as the same datatype in both the tables. N.B: Even if it is stored as a number, make sure they are both long integers. The duplicates shouldn’t prevent you from creating a relationship: duplicates restrict only when you try and create a relationship with referential integrity.