MySQL provides us with the WITH ROLLUP modifier for the GROUP BY function for generating super aggregate data.
But by default, this modifier doesn’t label its data. By this I mean, it doesn’t provide us with meaningful labels for the rows that it generates. It simply uses NULL in those rows.
Maybe you’ve encountered this and are now looking for a solution.
Below is a handy little technique we can use to replace NULL with our own headings for these rows.
If you get an error that reads “WRONGTYPE Operation against a key holding the wrong kind of value” when using the ZINTER command in Redis, it’s probably because you’re passing a key with the wrong data type.
To fix this issue, be sure that the key you pass to the ZINTER command holds a sorted set.
When we create or modify a table in MySQL, we have the option of applying a CHECK constraint against one or more columns. This allows us to check the data before it enters the database. Data can only enter the database if it doesn’t violate the rules in our CHECK constraint.
If the column is a JSON column, we have the option of creating a CHECK constraint that checks that the JSON document doesn’t violate its JSON schema. To do this, we can include the schema in the CHECK constraint.
If you’re getting an error in PostgreSQL that tells you that a column “…can only be updated to DEFAULT” with detail that explains that it “…is an identity column defined as GENERATED ALWAYS“, it’s probably because you’re trying to update an identity column with your own value, but the identity column was created with the GENERATED ALWAYS option.
As of MySQL 8.0.16, we can create CHECK constraints in MySQL.
A CHECK constraint is a type of integrity constraint. It specifies a search condition to check the value being entered into a row. If the value being entered violates the CHECK constraint, then the result of the search condition is FALSE and an error occurs (unless the IGNORE clause is used in the SQL statement, in which case a warning is reported and the offending row is skipped).
The ability to create CHECK constraints was introduced in MySQL 8.0.16. Prior to version 8.0.16, MySQL actually allowed a limited version of CHECK constraint syntax, but it was completely ignored (no CHECK constraint was created or evaluated).
If you’re getting an error that reads something like “nextval: reached minimum value of sequence “sequence1” (-3)” in PostgreSQL, it’s probably because you’re trying to generate a new value from a descending sequence, but the sequence has already reached its minimum value.
We have a number options when it comes to dealing with this issue.
Continue readingWhen learning SQL, a common mistake when checking for NULL values is to use an equality operator. In SQL, a NULL value is treated a bit differently to other values.
In SQL we use IS NULL instead of = NULL. Likewise, we use IS NOT NULL instead of <> NULL or != NULL to select those columns that don’t have a NULL value.
In MySQL, the data directory stores information managed by the MySQL server. Each subdirectory of the data directory is a database directory and corresponds to a database managed by the server.
If you ever need to find out where the data directory is located on your MySQL implementation, below are some options to try.
Continue readingIf you’ve been using MySQL for any decent amount of time, it’s likely you’ll be familiar with error 1055 that reads something like “Expression #N of SELECT list is not in GROUP BY clause and contains nonaggregated column…“, where #N is the expression number of an expression/column in your SELECT list.
This error can occur when we include a column in the SELECT list, but we omit it from the GROUP BY clause.
There are several ways we can go about resolving this issue. Below are six options for dealing with this issue.
Continue readingIf you’re getting error 1048 that reads something like “Column ‘ColumnName’ cannot be null” (where ColumnName is the name of a column you’re trying to insert data into), then it’s probably because you’re trying to insert a NULL value into that column, but the column has a NOT NULL constraint (which prevents NULL values from being inserted).
We have a few options when it comes to fixing this issue. The most obvious is to ensure we provide a non-NULL value for the column. Alternatively, if the column should be able to accept NULL values, then we can remove the NOT NULL constraint from the column. Another option is to use the IGNORE keyword to ignore the error. And another way to deal with the error is to disable strict mode.