List of user sessions in SQL

| category: Programming | author: st

The frequently asked problem on interviews.

You have a log of some user activities represented as a table. Every activity record has at least users ID and activity date/time values.

The session is a sequence of activities having less than N minutes between two log records. When the elapsed …

Should database only store data?

| category: Design | author: st
Tags: , ,

Very old subject I remember to discuss in the middle of 1990s... But some people still says that a clean architecture should remove all business logic from the database.

Let's start from referential and domain integrity rules (constraints). Are they business ones? Of course, yes. The e-mail column should be …

All expressions of 123456789 and signs +/- which value is 100

| category: Programming | author: st

Suppose a string of ordered cyphers 123456789. You can insert signs "+" and "-" between any cyphers to make a correct arithmetical expression. The problem is to find all expressions which sum is 100.

This problem may be interesting for dynamic script languages having the function of a string expression evaluation.

E …

SQL Server: key-value store table and hash index

| category: Testing | author: st
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Hash indexes are used as entry points for memory-optimized tables; both features were introduced in SQL Server 2014. Let's look at index performance in scenario "key-value store".

Scenario and environment

The table stores a significant volume (about 10 millions rows) of pairs key-value. The key is of integer type and …

SQL: generate random character string

| category: Programming | author: st

Collection of methods to generate random string in a single SQL statement

M1: Recursive string concatenation

On SQL Server the method is limited by 100 characters because of CTE recursion level limitation. The method has the performance issues when using in loops/joins because the table always have 100 rows …

SQL Server: sync two tables with MERGE statement

| category: Programming | author: st
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This small example shows how to synchronize the data of two tables using the MERGE statement

CREATE TABLE table1 (id int NOT NULL PRIMARY KEY, str_value nvarchar(50));
CREATE TABLE table2 (id int NOT NULL PRIMARY KEY, str_value nvarchar(50));
INSERT INTO table1 (id, str_value) VALUES (1, 'Value 1 …

SQL: generate date/time series

| category: Programming | author: st

There are many examples of user defined table functions generating the series of dates(times) between two specified values. Usually, in OLAP you can see a table filled by the date/time series, too. Such table avoid to generate the values "on the fly".

However, in some cases you may …

SQL quine (self-reproducing SQL)

| category: Programming | author: st

A quine is a computer program which takes no input and produces a copy of its own source code as its only output. The challenge for programmers is to write the shortest code in the given language. Here are my examples in Transact SQL and in ANSI SQL.

Note that …