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The SQL problems studied in the posts on this site can become a serious trouble when updating a database; I mean update in terms of insert, delete and update as such a database.

There are several possibilities in technology at the time of updating a database, for example, we have the case of input data to an information system, when we are migrating a database to a new one that usually has a different structure, in the case of updating a distributed databases that are spread in different locations, when we are loading data to a Data Mart, and a number of other possibilities, in all of them we must be aware of the danger of inserting data into a database when we don’t know the weaknesses of SQL.

In this case the problems go beyond query results, we know that the only way to update a SQL database is by using the language SQL, in the case of an information system a simple interface could let us put data in it which could erroneous data in database, from there on we could never trust in the query results anymore The possibility of erroneous data in SQL databases is present with standard operations INSERT, UPDATE and DELETE.

These operations may cause incorrect data inserted only in the case of using the Table expressions in the query, I mean using the SELECT expression within the operations INSERT, UPDATE and DELETE.

Essential to know about the weaknesses SQL have.

A detailed explanation plus some other problems found with the SQL aggregate operators is found in my book:

When subquery is empty in EXISTS (subquery)

Take a look at the example:

Let’s analyse the result of the second example in the mentioned link:

The query: “Get the Bank names which doesn’t have ATMs in Madrid”

The result of the EXISTS subquery is empty, I mean that the subquery generates no tuples because the only possible tuple is the one that has Madrid in the attribute City an null in the attribute BNK, and the condition NOT EXISTS (subquery) generates True, that’s why we get a result with all the Bank names from de table BANK.

Then, how does the condition EXISTS behave from the perspective of SQL? Is this behaviour the same as in Logic?.

A detailed explanation is found in my book:

The Publication

These examples above are only three of the weaknesses that SQL has on database processing, in my publication you can find a complete analysis on the following points related to the weaknesses and conflictive aspects of SQL:

  1. The Interpretation of null
  2. The Relational Algebra Operations
  3. The conditions EXISTS, IN, ANY and ALL
  4. The Aggregation Operators
  5. The Implication
  6. Operations INSERT, UPDATE and DELETE

you’ll find all the information needed to get this essential work about the language SQL at these links:


The publication at:



Contact me at::