NULL

A LEFT JOIN keeps every row from the first table even when nothing matches on the second. Where nothing matches, SQL doesn't leave the cell blank or put in a zero: it fills it with NULL, SQL's word for “nothing recorded here.”

SELECT st.name, s.section
FROM staff st
LEFT JOIN stations s ON st.name = s.lead;

st.name       s.section
Giorgos       hot
Yannick       cold
Sofia         cold
Dan           hot
Konstantinos  NULL
Eleni         NULL
Marina        NULL

You can't filter for NULL with = NULL: to SQL, nothing ever “equals” nothing, so that comparison silently matches zero rows. Use IS NULL instead, or IS NOT NULL for the opposite.

SELECT st.name
FROM staff st
LEFT JOIN stations s ON st.name = s.lead
WHERE s.section IS NULL;

st.name
Konstantinos
Eleni
Marina

NULL printed on a real docket looks like a bug, not an answer. COALESCE swaps it for a fallback value you choose, and leaves anything that isn't NULL exactly as it was.

SELECT st.name, COALESCE(s.section, 'Not assigned') AS section
FROM staff st
LEFT JOIN stations s ON st.name = s.lead
ORDER BY st.name;

st.name       section
Dan           hot
Eleni         Not assigned
Giorgos       hot
Konstantinos  Not assigned
Marina        Not assigned
Sofia         cold
Yannick       cold

One more habit worth knowing: COUNT(*) counts every row, but COUNT(some_column) only counts the rows where that column isn't NULL. Here that difference is the whole answer: 4 staff are assigned to a station, and 3 aren't, all from one join.