Wildcards in column name for MySQL

Question

I am trying to select multiple columns, but not all of the columns, from the database. All of the columns I want to select are going to start with "word".

So in pseudocode I'd like to do this:

SELECT "word%" from searchterms where onstate = 1;

More or less. I am not finding any documentation on how to do this - is it possible in MySQL? Basically, I am trying to store a list of words in a single row, with an identifier, and I want to associate all of the words with that identifier when I pull the records. All of the words are going to be joined as a string and passed to another function in an array/dictionary with their identifier.

I am trying to make as FEW database calls as possible to keep speedy code.

Ok, here's another question for you guys:

There are going to be a variable number of columns with the name "word" in them. Would it be faster to do a separate database call for each row, with a generated Python query per row, or would it be faster to simply SELECT *, and only use the columns I needed? Is it possible to say SELECT * NOT XYZ?

Solution 1

No, SQL doesn't provide you with any syntax to do such a select.

What you can do is ask MySQL for a list of column names first, then generate the SQL query from that information.

SELECT column_name
FROM information_schema.columns
WHERE table_name = 'your_table'
    AND column_name LIKE 'word%'

let's you select the column names. Then you can do, in Python:

"SELECT * FROM your_table WHERE " + ' '.join(['%s = 1' % name for name in columns])

Instead of using string concatenation, I would recommend using SQLAlchemy instead to do the SQL generating for you.

However, if all you are doing is limit the number of columns there is no need to do a dynamic query like this at all. The hard work for the database is selecting the rows; it makes little difference to send you 5 columns out of 10, or all 10.

In that case just use a "SELECT * FROM ..." and use Python to pick out the columns from the result set.

Solution 2

No, you cannot dynamically produce the list of columns to be selected. It will have to be hardcoded in your final query.

Your current query would produce a result set with one column and the value of that column would be the string "word%" in all rows that satisfy the condition.

Solution 3

You can generate the list of column names first by using

SHOW COLUMNS IN tblname LIKE "word%"

Then loop through the cursor and generate SQL statement uses all the columns from the query above.

"SELECT {0} FROM searchterms WHERE onstate = 1".format(', '.join(columns))

Comments

[Steven Matthews]

Wouldn't this, in practical terms, give me the exact list I had to use?

[Martijn Pieters]

Yes, but you'll have to use Python to then construct the final SQL; there is no pure SQL way of implementing your psuedocode query.