sql tagged templates interpolates variable names, not values

[iconara]

This code ends up running SELECT DISTINCT columnName FROM my_table (it interpolates the name of the variable into the SQL):

const columnName = 'my_column'
const rows = await sql`SELECT DISTINCT ${columnName} FROM my_table`
// runs SELECT DISTINCT columnName FROM my_table

I expected it to run SELECT DISTINCT my_column FROM table (interpolate the value of columnName into the SQL). Variable interpolation seems to work this way in fenced sql blocks, and the following code also works:

const columnName = 'my_column'
const rows = await sql(['SELECT DISTINCT ' + columnName + ' FROM my_table'])
// runs SELECT DISTINCT my_column FROM my_table

but this, again, does not

const columnName = 'my_column'
const rows = await sql(['SELECT DISTINCT ', ' FROM my_table'], columnName)
// runs SELECT DISTINCT columnName FROM my_table

[mbostock]

You can’t interpolate SQL fragments into queries, you can only interpolate values. Your original query is equivalent to

SELECT DISTINCT ? FROM my_table

where the value of ? is passed separately as the string value "my_column". If you want to build up a SQL query dynamically (rather than just passing in dynamic values), you’ll need to construct the string yourself.

[iconara]

Oh, I see. I didn't realize the sql function rewrote the arguments like a prepared statement. That explains it. It means it's the classic issue with prepared statements, they can't be used to parameterize column and table names. Thank you for the explanation.