Finding Duplicates with SQL. To identify duplicates, I use this set of queries Identify duplicates on one key Gives the list of the keys that have a duplicated recordselect key1, count from table1group by key1having count 1 Same as above but with multiple keys searchselect key1, key2,., keyn count from table1group by key1, key2,., keynhaving count 1 Match the duplicated keys to their matching record in the tableselect from table1where key1 in select key1, count from table1group by key1having count 1 Same as above with multiple key searchselect from table1where key1, key2,., keyn in select key1, key2,., keyn, count from table1group by key1, key2,., keynhaving count 1 Associate duplicated record by their unique identifier on a single row Requires to have an unique identifier on the table analysed a primary key for instance Will return cyclic results if the duplicated record count exceeds two results per keyselect s. GROUP BY key1having count 1 S1, select uniqueidentifier, key1 from table1 where key1 in select key1 from select key1, countfrom table1. GROUP BY key1having count 1 S2 where S1. Same as above with multiple keys search and composed unique identifierselect s. GROUP BY key1, key2,., keynhaving count 1 S1, select primarykeyfield1, primarykeyfield2,., primarykeyfieldn,key1, key2,., keyn from table1 where key1, key2,., keyn in select key1, key2,., keyn from select key1, key2,., keyn, countfrom table1. GROUP BY key1, key2,., keynhaving count 1 S2,where S1. S1. key2 s. 2. S1. S1. keyn s. 2. S1. S1. S1. S1. primarykeyfieldn lt S2. S2. primarykeyfield2S2. S2. Implicit conversion may sometimes fails according to the primarykeyfield types. You may then use conversion fuctions to make them all match the same type. Images/1637_MSSQLT2.jpg' alt='Sql Update Table Remove Duplicates Access' title='Sql Update Table Remove Duplicates Access' />Unique key Wikipedia. In database relational modeling and implementation, a unique key1 is a superkeythat is, in the relational model of database organization, a set of attributes of a relation variable for which it holds that in all relations assigned to that variable, there are no two distinct tuples rows that have the same values for the attributes in this set. When more than one column is combined to form a unique key, their combined value is used to access each row and maintain uniqueness. These keys are referred to as aggregate or compound keys. Values are not combined, they are compared using their data types. When a column or set of columns is defined as unique to the database management system, the system verifies that each set of values is unique before assigning the constraint. After the columns isare defined as unique, an error will occur if an insertion is attempted with values that already exist. Some systems will not allow key values to be updated, all systems will not allow duplicates. This ensures that uniqueness is maintained in both the primary table and any relations that are later bound to it. SummaryeditKeys provide the means for database users and application software to identify, access and update information in a database table. There may be several keys in any given table. Different%20strategies%20for%20handling%20duplicates%20in%20SQL%20Server%20-%202.jpg' alt='Sql Update Table Remove Duplicates Access' title='Sql Update Table Remove Duplicates Access' />This tutorial explains in great depth how the SQL language works. It has many examples and uses access databases to demonstrate the SQL syntax. I have 2 tables a main APPLICATION table that holds the core data, and a STATUSTRACKING table that reflects the status changes of the core data in the APPLICATION table. For example, two distinct keys in a table of employees might be employee number and login name. The enforcement of a key constraint i. The DBMS prevents updates that would cause duplicate key values and thereby ensures that tables always comply with the desired rules for uniqueness. Proper selection of keys when designing a database is therefore an important aspect of database integrity. A relational database table may have one or more available keys formally called candidate keys. One of those keys per table may be designated the primary key, alternatively another key surrogate key may be used. Any remaining keys are called alternate keys. Although mainly used today in the relational database context, the term primary key pre date the relational model and is also used in other database modelscitation needed. In relational database terms, a primary key need not differ in form or function from a key that isnt primary and in practice various different motivations may determine the choice of any one key as primary over another. The designation of a primary key may indicate the preferred identifier for data in the table, or that the primary key is to be used for foreign key references from other tables or it may indicate some other technical rather than semantic feature of the table. Some languages and software have special syntax features that can be used to identify a primary key as such e. PRIMARY KEY constraint in SQL. Any key may consist of one or more attributes. For example, a Social Security Number might be a single attribute key for an employee a combination of flight number and date might be a key consisting of two attributes for a scheduled flight. There are several types of keys used in database modeling and implementations. Key Name. Definition. Auto Diagnose Software Download Freeware. Simple. A key made from only one attribute. Concatenated. A key made from more than one attribute joined together as a single key, such as part or whole name with a system generated number appended as often used for E mail addresses. Compound. A key made from at least two attributes or simple keys, only simple keys exist in a compound key. Composite. A key containing at least one compound key with at least one other attribute or simple key this is an extension of a compound key. Natural. A key made from data that exists outside the current database. In other words, the data is not system generated, such as a social security number imported from another system. Surrogate. An artificial key made from data that is system assigned or generated when another candidate key exists. Surrogate keys are usually numeric ID values and often used for performance reasons. Candidate. A key that may become the primary key. Primary. The key that is selected as the primary key. Only one key within an entity is selected to be the primary key. This is the key that is allowed to migrate to other entities to define the relationships that exist among the entities. When the data model is instantiated into a physical database, it is the key that the system uses the most when accessing the table, or joining the tables together when selecting data. Alternate. A non primary key that can be used to identify only one row in a table. Alternate keys may be used like a primary key in a single table select. Foreign. A unique key that has migrated to another entity. At the most basic definition, a key is a unique identifier,3 so unique key is a pleonasm. Keys that are within their originating entity are unique within that entity. Keys that migrate to another entity may or may not be unique, depending on the design and how they are used in the other table. Foreign keys may be the primary key in another table for example a Person. ID may become the Employee. ID in the Employee table. In this case, the Employee. ID is both a foreign key and the unique primary key, meaning that the tables have a 1 1 relationship. In the case where the person entity contained the biological father ID, the father ID would not be expected to be unique because a father may have more than one child. Here is an example of a primary key becoming a foreign key on a related table. ID migrates from the Author table to the Book table. Author. Table. Schema AuthorID,Name,Address,BornBook. Table. Schema BookISBN,Author. ID,Title,Publisher,PriceHere ID serves as the primary key in the table Author, but also as Author. ID serves as a Foreign Key in the table Book. The Foreign Key serves as the link, and therefore the connection, between the two related tables in this sample database. In a relational database, a candidate key uniquely identifies each row of data values in a database table. A candidate key comprises a single column or a set of columns in a single database table. No two distinct rows or data records in a database table can have the same data value or combination of data values in those candidate key columns since NULL values are not used. Depending on its design, a database table may have many candidate keys but at most one candidate key may be distinguished as the primary key. A key constraint applies to the set of tuples in a table at any given point in time. A key is not necessarily a unique identifier across the population of all possible instances of tuples that could be stored in a table but it does imply a data integrity rule that duplicates should not be allowed in the database table. Some possible examples of unique keys are Social Security Numbers, ISBNs, vehicle registration numbers or user login names. The relational model, as expressed through relational calculus and relational algebra, does not distinguish between primary keys and other kinds of keys. Primary keys were added to the SQL standard mainly as a convenience to the application programmer. Unique keys as well as primary keys may be logically referenced by foreign keys, but most RDBMS only allow a foreign key constraint against a primary key. Defining primary keys in SQLeditPrimary keys are defined in the ANSI SQL Standard, through the PRIMARY KEY constraint. The syntax to add such a constraint to an existing table is defined in SQL 2.