If you’ve got a table with an invisible column in MySQL, but you no longer want the column to be invisible, you can use the ALTER TABLE statement to make it visible.
Invisible columns are columns that can’t be accessed by queries that use the asterisk wildcard (*) to select all columns (although they can be accessed by explicitly naming the invisible column in the SELECT list).
In MySQL we can use the DESCRIBE statement to get information about table structure in the database, or to obtain a query execution plan for a query. For example, we can get information about column names, their data types, default values, etc.
The DESC and EXPLAIN statements are synonyms of DESCRIBE, so we can alternatively use those to do the same thing.
If you’re getting an error that reads “This version of MySQL doesn’t yet support ‘EXPLAIN ANALYZE FOR CONNECTION’” in MySQL, it’s probably because you’re trying to use the FOR CONNECTION option with EXPLAIN ANALYZE, but EXPLAIN ANALYZE doesn’t support the FOR CONNECTION option.
It’s true that we can use the FOR CONNECTION option with the EXPLAIN statement, we can’t use it when using the ANALYZE variant of EXPLAIN (we can use EXPLAIN with or without ANALYZE). At least, this is how it works as of this writing (MySQL 8.3).
To fix, either remove the ANALYZE keyword from the statement or don’t use the FOR CONNECTION option.
Since MySQL 8.0.23 we’ve had the ability to create invisible columns, which are columns that are normally hidden to queries (but can easily be accessed if required).
With the introduction of invisible columns comes the inevitable requirement to check a table to see if it contains any invisible columns.
Fortunately there are many ways we can do this. Below are five ways to check a table for invisible columns.
In MySQL we can use the SHOW COLUMNS statement to get information about the columns in a table or view.
We can narrow the columns down to just a certain table and database. We can also filter by column name using a pattern.
The SHOW COLUMNS statement only shows information about columns that we have some privilege.
In MySQL 8.0.23 and later, we have the ability to create invisible columns when creating or altering a table.
An invisible column is one that is hidden to certain SQL queries, but can be accessed if explicitly referenced.
Basically, invisible columns are hidden to queries that use the asterisk wildcard (*) in their SELECT list. But they can be accessed by explicitly referencing the column by its name.
Invisible tables are hidden from the TABLE statement, as this statement provides no way of selecting individual columns.
If you’re getting an error that reads “The function ‘RANK’ must have an OVER clause” in SQL Server, it’s probably because you’re calling the RANK() function without an OVER clause.
The RANK() function requires an OVER clause (and that clause must have an ORDER BY clause).
To fix this issue, add an OVER clause when calling the RANK() function.
MySQL has the handy TABLE statement that allows us to get the contents of a table with less code than what we’d need if we’d used the SELECT statement.
When we use the TABLE statement, it returns the columns using the actual column names. In most cases this is fine, and there’s no need to change anything. But sometimes we might want to rename one or more columns. In such cases, we can use the following techniques to achieve that outcome.
When combining multiple query blocks in MySQL, we can use the EXCEPT clause to return just those rows from the first block that are not present in the second block.
If you’re getting an error that reads “The used SELECT statements have a different number of columns” in MySQL, it’s probably because you’re using the EXCEPT, INTERSECT, or UNION clause, but with a different number of columns for each query.
When we use these clauses, both queries must select the same number of columns. For example, if the first query has two columns in its SELECT list, then the second query must also have two columns in its SELECT list.
To fix, make sure both queries select the same number of columns.