How to ST_contains two gis layers (lines and polygon) and updatelines with spatialite-gui

Question

I'm a biginner of spatialite-gui.

I have 300,000 lines and one polygon, this polygon contains some of the lines.

So, I tried to use function (ST_contains) to add a flag (999) to the target lines and update them. But I can't do it well. I would be happy to get some advice.

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update table set flug = 999 from lines as l polygon as p where ST_contains(l.geom, p.geom)=1;