In Redis, the LPUSH command inserts one or more values into the head of the list at the specified key.
If the key doesn’t exist, it is created as an empty list before performing the push operation. If the key already holds a value that is not a list, an error is returned.
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.
The 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.
PostgreSQL has supported generated columns since version 12. Generated columns can be created easily with the CREATE TABLE and ALTER TABLE statements.
We can check whether a table contains a generated column or not with the following methods.
When setting a timeout for a key in Redis, we have the option of setting it only if the key already has an expiry. So, if the key doesn’t already have an associated timeout, the new timeout won’t apply. The new timeout will only apply if the key already has an existing timeout. In such cases the expiry will be reset to the new expiry.
We can do this with the XX option of the EXPIRE command. We can also use the XX option on commands such as EXPIREAT, PEXPIRE, and PEXPIREAT.
If you’re trying to add a generated column in SQLite, and you’re getting the “cannot add a STORED column” error, it’s probably because you’re trying to add a stored column.
In SQLite, we can only add virtual columns to existing tables.