Database Management Systems
This 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 PostgreSQL, the COALESCE() operator returns the first non-null value from its list of arguments. If all arguments are null, it returns null.
In SQLite, we can use the TIME() function to add a given number of seconds to a time value.
If we’re dealing with datetime values, we can use the DATETIME() function.
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.
Below are two options for removing duplicate rows from a table in PostgreSQL when those rows have a primary key or unique identifier column. The primary key is used in the query, but it’s ignored when comparing duplicates (due to the fact that primary keys prevent duplicate rows by definition).
The following examples delete duplicate rows but keep one. So in the case of say, three identical rows, it deletes two and keeps one.
Continue readingIn PostgreSQL, we can use the - operator to subtract one or more hours from a time value.
By “time” value, this could be an actual time value, a timestamp, or an interval. We can also subtract hours from a date value or a date and time combination.
Below are two Oracle Database functions that convert uppercase characters to their lowercase equivalent.
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