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Can we have a MySQL table without primary key?
Table does not have to have primary key. Much more important is to have correct indexes. On database engine depends how primary key affects indexes (i.e. creates unique index for primary key column/columns).
Is it possible for an entity to not have a primary key?
Every entity in the data model shall have a primary key whose values uniquely identify entity instances. The primary key attribute cannot be optional (i.e., have null values).
Can you do a Vlookup in SQL?
The most used function of excel by any BI professional / data analyst is VLOOKUP(). It helps to map data from other table to parent table. In other words, we can say that it is the ‘excel’ way of joining 2 data sets through a common key. In SQL, we have similar functionality known as JOIN.
How to find table without primary key in MySQL?
The query below lists tables in the user databases (schemas) without primary keys. Note: if you need the information for a specific database (schema), then uncomment the table_schema line and provide your database name. There are no comments. Click here to write the first comment.
Can a primary key be a composite of two values?
I would not make the primary key of the “info” table a composite of the two values from other tables. Others can articulate the reasons better, but it feels wrong to have a column that is really made up of two pieces of information. What if you want to sort on the ID from the second table for some reason?
Can a foreign key be a composite key?
This way you can have t1ID and t2ID as foreign keys pointing to their respective tables as well. I would not make the primary key of the “info” table a composite of the two values from other tables. Others can articulate the reasons better, but it feels wrong to have a column that is really made up of two pieces of information.
Can a table have more than one foreign key?
Tables may have two or more fields that provide a unique combination, and not necessarily by way of foreign keys. As an example, each US state has a set of unique Congressional districts. While many states may individually have a CD-5, there will never be more than one CD-5 in any of the 50 states, and vice versa.