Tag: how to

Fix Error 1138 “Invalid use of NULL value” in MySQL

Posted on by Ian

If you’re getting error 1138 that reads “invalid use of NULL value“, it could be because you’re trying to change the definition of a column to require non-NULL values, but the column contains NULL values.

This can happen when we try to change a column from NOT NULL to NULL. It can also happen when we try to set a column as a primary key, but that column contains NULL values.

To fix this issue, be sure to ensure that the column you’re modifying doesn’t contain any NULL values. Alternatively, if the column should be able to accept NULL values, then you should change your column definition to allow NULL values.

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How to Add Headings for WITH ROLLUP in MySQL

Posted on by Ian

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.

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How to Create a CHECK Constraint Against a JSON Column in MySQL

Posted on by Ian

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.

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Fix “column … can only be updated to DEFAULT” in PostgreSQL

Posted on by Ian

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.

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Create a CHECK Constraint in MySQL

Posted on by Ian

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).

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6 Ways to Fix “nextval: reached minimum value of sequence” in PostgreSQL

Posted on by Ian

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.

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