We can use SQLite’s DATE() function to return the first, second, third, fourth, etc instance of a given day within a given year.
We can use the same technique to return the first Monday of each month in the year.
It’s not limited to Monday though. The same techniques can be used on Tuesday, Wednesday, Thursday, Friday, etc.
Continue readingWhen setting an expiry for an existing key in Redis, we now have the option to set the expiry only if the key doesn’t already have an expiry. That’s because Redis 7.0.0 introduced some new options that allows us to do this and other things.
In this case, we can use the NX option when setting the expiry to specify that the expiry should only be set if the key doesn’t already have an expiry. We can use this option on commands such as EXPIRE, EXPIREAT, PEXPIRE, and PEXPIREAT.
In PostgreSQL, we can use the + operator to add one or more seconds to a time value.
By “time” value, this could be an actual time value, a timestamp, or an interval. We can also add seconds to a date value or a date and time combination.
A generated column is a database column whose value is an expression. Therefore, the value of the generated column is usually derived (for example, it could be derived from values in other columns).
Generated columns are also sometimes referred to as computed columns or virtual columns.
Creating a generated column in MariaDB is essentially the same as creating a normal column. The difference is that the definition of the generated column contains an expression that determines the column’s value.
Continue readingWhen using Redis, we often have many ways to do the same thing. One example of this is with deleting keys.
Below are four options for deleting keys in Redis.
Continue readingIn SQLite, we can use the TIME() function to subtract one or more seconds from a time value.
For datetime values, we can use the DATETIME() function.
In Redis, we can use the STRLEN command to get the length of a given string value, based on its key.
We can also use the HSTRLEN command to get the string length of a value stored in a hash.
Redis provides us with quite a few options for setting keys and any timeouts associated with those keys. Sometimes we can do it with a single command, other times we need two commands.
Here are four ways to set a key and its expiry in Redis.
Continue readingThe following examples use SQL to delete duplicate rows that have a primary key or unique identifier column.
Specifically, the examples delete all duplicates except one. So for example, in the case of three identical rows, it deletes two of them and keeps one.
Continue readingIf we need to append a value to a string in Redis, we can use the APPEND command to do just that. All we need to do is call the command with the key name and the value to append.