Here are examples of converting a Unix timestamp to a date/time value in some of the major RDBMSs.
The Unix timestamp (also known as Unix Epoch time, Unix time, or POSIX time) is the number of seconds that have elapsed since 00:00:00 Thursday, 1 January 1970, Coordinated Universal Time (UTC).
Continue readingThis article contains an example of adding a generated column to a table in MySQL.
Also known as computed columns, generated columns usually contain values that are dependent on other factors (such as the values in other columns).
Creating (or adding) a generated column in MySQL is basically the same as creating a normal column, except that the definition of the generated column contains an expression that determines the column’s value.
Continue readingAlphanumeric characters are alphabetic characters and numeric characters.
Below are examples of using SQL to return rows that contain only alphanumeric characters.
Continue readingHere are three methods we can use to return the first day of a given month in MySQL.
This could be the first day of the current month, or the first day of the month based on a date that we specify.
Getting the first day of the month allows us to perform further calculations on the resulting date, like adding a certain number of days to the start of the month, etc.
Continue readingSome RDBMSs provide an ISNULL() function that can be used when dealing with potentially null values.
MySQL, MariaDB, and Oracle Database each have an ISNULL() function that returns 1 if its argument is null, and 0 if it’s not.
SQL Server also has an ISNULL() function, but it works differently. It works more like how the IFNULL() function works in some other RDBMSs.
Other RDBMSs, such as PostgreSQL and SQLite don’t include an ISNULL() function, but they do support the IS NULL predicate (as do the other RDBMSs).
In MySQL, the CASE statement can be used in stored programs to perform a complex conditional construct. It compares a list of conditions and returns a different result depending on which condition (if any) is matched.
The CASE statement is different to the CASE operator, in that the CASE statement is specifically for use in stored programs. Also, there’s a slight difference in the syntax.
In MySQL, the CASE operator compares a list of conditions and returns one of multiple possible result expressions.
The CASE expression is included in the SQL standard (ISO/IEC 9075), and most major RDBMSs support it.
MySQL also has the CASE statement, which is slightly different to the CASE operator. This article is about the CASE operator.
Below are examples of using some of the more popular RDBMSs to return the Unix timestamp.
The Unix timestamp (also known as Unix Epoch time, Unix time, or POSIX time) is the number of seconds that have elapsed since 00:00:00 Thursday, 1 January 1970, Coordinated Universal Time (UTC).
Continue readingSome RDBMSs provide an IFNULL() function that can be used when dealing with potentially null values. In particular, MySQL, MariaDB, and SQLite each have an IFNULL() function that replaces any null values with another value.
Other RDBMSs, such as SQL Server, Oracle, and PostgreSQL provide similar functionality via functions of a different name.
Continue readingIf you ever encounter a character column that should be numeric, there’s always a possibility that it contains non-numeric data that you don’t know about.
For example, someone might have set up a Price column as a varchar column that should have been a numeric column, and now you need to clean up after them. You might start by identifying all non-numeric data so that you can work out what to do with it before converting the column to a numeric type.
In SQL, you can run a query to return non-numeric data from the column. The query you use will largely depend on your DBMS.
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