I have a two polyline layers - one with 2800 rows, the other with 150 rows. Each row has a unique id column. I need to replace the records in one column of the the 2800-row layer with the records of the same column in the 150-row layer by id. What would be the fastest method to do this?
If you join by
id, as mentioned by @raphael, you could use the following expression in the Field Calculator which would replace values from the old column with those in the joined column. And if there's a NULL in the joined column, the old column will keep its original value:
if( "JoinColumn" IS NULL, "Column", "JoinColumn" )
If you join the two layers by your unique ID, you will be left with one layer with 150 records and 2x the columns. In the attribute table just delete the extraneous columns and your old column. Then rename the new column to the old column name and save this layer over your old one and you should be golden.