Detect Overlapping Intervals

Let’s see how Query Xtractor can help you solve the typical SQL problem of detecting overlapping datetime intervals. We have a table of projects employees are working on for a period of time, between a start and end date. It is indeed very hard to figure out from the results grid how large each period of time is and how they compare to each other.

Why BETWEEN is error prone

We do not use BETWEEN for now, and chances are we’ll never expose it as a direct operator in our query builder. Sure, there are pros and cons on using x BETWEEN a AND b rather then x >= a AND x <= b, and that’s one reason why we have this article. However, assuming our tool tries to avoids as much as possible error prone SQL constructs and the “element of surprise”, it seems like the bad won over the good. (more…)

Case Study: Planning for Database Migrations

Many project managers, database administrators, system architects or software developers experience at least once in their lifetime the perspective of a database migration. Your company acquired Oracle and you want to see if your SQL Server based application cannot be easily switched to Oracle. Or to an open-source client-server database such as MySQL or PostgreSQL. Or, for a small application with no justification for such large resources, you want to check the cost of migrating all your queries to SQLite or Firebird. (more…)

Learn about ALL and SOME/ANY

Query Xtractor is mainly a productivity tool. But it can be also used to learn SQL. One particularly great way to do it is to turn ON one of the Always Emulate options. We have this for grouping sets, crosstabs, Intersect/Except, All/Some/Any. With the setting OFF, we use the native support when available, and we emulate features when there is no other way. The obvious advantage in having ALL/SOME/ANY expressions emulated as well for let’s say SQL Server – where they are recognized and implemented by the vendor – is to better understand what they actually do through equivalent constructs. (more…)

Intersect and Except

Not all database platforms support INTERSECT and EXCEPT (MINUS in Oracle), and MySQL is one of them. Let’s try to solve two simple related problems:

  • We want department numbers that appear in both Departments and Employees tables. In other words, we’re looking for all departments with at least one employee. This is a great candidate for INTERSECT, because we want numbers that exist in both tables.
  • We also want all department numbers that do not appear in Employees. In other words, we’re looking for departments with no employees. This is a great candidate for EXCEPT, because we want numbers that exist in only one table.