SOLVED! “No Unique Index Found For The Database Reference Field” Error: Analysis

This frequent error appears when we build the intra table relation with reference fields. If the reference field is not unique, the DBMS engine fails to build referential integrity and throws the error “No unique index found for the Database reference field.” We need to realize how the Database engine interprets the table relationship to fix this error.

Database Management System (DBMS) and Intra Table Relation:

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The Database Management System runs based on intra table relation. A foreign key reference entirely controls the linkage between two tables. This foreign key enforces referential integrity between multiple tables. It is mandatory to have a unique index field in one relationship table to establish the referential integrity. DBMS will not enforce the referential integrity between tables if the unique index is missing. 

The unique index of referenced fields strictly controls the referential integrity of all the modern Database Management systems like Microsoft Access, Oracle, SQL Server Management Studio. However, looking into the Distributed File System Database such as Hadoop, Bigdata, etc., it’s not very obvious to have referential integrity between referenced tables. The database relation works in a completely different way and algorithm.

What is referential integrity?

Referential integrity defines the relationship between multiple tables. It is a logical dependency of a foreign key on a primary key. The integrity of a row that contains the foreign key depends on the integrity of the row which contains the matching primary key in another table. This referential integrity ensures the logical implementation of a relational database and enhances security. For example, when a transactional SQL tries to modify or delete a record from Database, it primarily checks if any referential integrity exists for the target record. If referential integrity exists, it forces the user to remove that integrity before making any T-SQL execution. 

Why is Unique Index Required for DB reference?

Database reference works with the uniqueness of the referred field. If the referred field is not unique, DBMS will fail to make the 1:1 binding of two records from both the table of a reference. For example, consider a table with a list of Countries and another table of List of Currency. Now for each country, the currency is unique. Suppose the country table contains multiple records with the same country name then, DBMS will fail to integrate the currency table record with a unique record from the country table and throws the error: No Unique Index Found for The Database Reference Field. Most of the time, the reference exists between a Primary Key and a Foreign Key. 

The primary key is by default unique in nature, and that’s why the reference goes smooth. However, if we use a different field other than the Primary Key for reference, the field must be unique. As per DBMS, the Unique Index must be enabled, which will ensure the uniqueness of the values that resides on each record of that column. So, in summary, we can say a unique index controls the authenticity of the relationship between multiple tables and enforces the logical access of records in the Database Management algorithm. Unique index in database relation also ensures all its references are valid. In a relational database, it implies a rule for the existence of a referenced record if it’s defined with referential integrity. 


How to fix “No Unique Index Found Error” in DBMS?

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The “No Unique Index Found Error” often appears while designing database architecture. A complex database structure consists of thousands of tables and their referential relation. The relation could be of the following types: one to one, many to many, one to many. Each of these relationships is entirely dependent on the implementation of referential integrity. And referential integrity works in the logic of a unique Index. To fix the “No Unique Index Found Error,” we can take several approaches as follows:

  • Try to use the Primary Key field and foreign key field to establish the relation of reference between two tables. By default, having a unique Index, the primary key is turned on, which helps to build a successful intra table relation.
  • Suppose the Primary key field may not have the unique index turned on in some situations. If we use the primary key field and still get the “No Unique Index Found Error,” we first need to check the field’s data type and ensure the unique index feature is turned on. 
  • Sometimes the datatype of the field may cause the issue. The reference field doesn’t like complex data types such as varchar(max), blob, clob, time-series, etc. Try to avoid these data types on the unique index field.
  • If we can’t use the Primary key for referencing, we must create a new unique field or declare some of the existing fields unique and then use the same field to define the table relation.
  • If you are already using a Unique field to define the relationship and still hit the error, it could be because of the nature of the data that field contains. It’s tricky to get rid of this type of situation. In this scenario, you can enable a unique Index on a combination of fields. In simple words, enable a unique key on a pair of columns and use that as a reference field to define a relationship. 
  • The other potential issue could be duplicate data in the reference field. If a duplicate record exists in the reference field, it throws the ‘Unique Index Not found Error.’ This is another tricky situation where you need to scan the entire table record and eliminate the duplicate entries. This type of error can be mitigated using the data manipulation language (DML).

Final Words

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Referential integrity is the backbone of the Database Management System. The reliability of data, security of data, and access of data are defined in database relation through referential integrity. Hence, all DBMS products prioritize defining a robust process of referential integrity controlled by the Unique index fields in the Database. Hence “No Unique Index Found for The Database Reference Field” can’t be considered an error; instead, it’s a mechanism to define the algorithm of table relation in DBMS. This validation controls the data structure in a Database.


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