The Grouparoo Blog


Are MySQL columns names sometimes case sensitive?

Tagged in Engineering Notes 
By Brian Leonard on 2021-05-26

There is a debate among a very specific set of people about what case to use in SQL queries. This debate is made possible by the fact that, generally, it does not matter. I believed this to be true even about identifiers like columns names.

For example, both of these queries returns the same data even though the "real" column is defined in lowercase.

  • SELECT first_name FROM users
  • SELECT FIRST_NAME FROM USERS

Let's try in the MySQL console.

mysql> SELECT first_name FROM users;
+------------+
| first_name |
+------------+
| John       |
| Sarah      |
+------------+
2 rows in set (0.00 sec)
mysql> SELECT FIRST_NAME FROM USERS;
+------------+
| FIRST_NAME |
+------------+
| John       |
| Sarah      |
+------------+
2 rows in set (0.00 sec)

Notice that the data is the same, but the "header" is different in each case.

So let's say I had code like this:

connection.query(
  "SELECT first_name FROM users",
  function (error, results, fields) {
    console.log("The user is: ", results[0].first_name);
  }
);

This would output The user is: John as expected. However, if the uppercase query was used (SELECT FIRST_NAME FROM USERS) instead, it would output The user is: undefined.

So you have to be a little careful here, but often, the same code is doing the query and using it. Its tends to work out.

But not always?

Our tool, Grouparoo connects to sources like MySQL to learn about customers and sync that data to destinations like Salesforce. In this process, we have to introspect the database.

We do a query that looks something like this:

mysql> SELECT column_name FROM INFORMATION_SCHEMA.COLUMNS
    ->   WHERE table_schema = 'db' AND table_name = 'users';
+-------------+
| COLUMN_NAME |
+-------------+
| first_name  |
| id          |
+-------------+
2 rows in set (0.00 sec)

Do you see what has happened here? I asked for lowercase column_name like before, but I got back uppercase COLUMN_NAME.

I have not been able to figure what is going on other than to assume the INFORMATION_SCHEMA is just different. I previously believed it was the same as all other tables in all respects.

We had only encountered lowercase values returned, but then ran across this one with a new user. Our code with row.column_name failed because it was undefined.

So what to do?

Having seen this, I was lacking the confidence to assume it was either way. The solution that I came up in the pull request was to be more explicit.

The code now does this query:

mysql> SELECT column_name AS column_name FROM INFORMATION_SCHEMA.COLUMNS
    ->   WHERE table_schema = 'db' AND table_name = 'users';
+-------------+
| column_name |
+-------------+
| first_name  |
| id          |
+-------------+
2 rows in set (0.00 sec)

I don't feel like this should be necessary, but you can't argue with (database) results.

Now, the row.column_name works as expected and we can help users pick the column(s) they want to sync to their tools.

Some sensitive cases

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