Sort and filter · Lập trình với Martin

A filter hides. It does not delete.

You have twenty survey rows and you only want to see “apple”. The filter hides the rest. After you turn it off, they come back. That is not an eraser.

Sorting shuffles the order of the rows. The whole row must travel together — subject and grade, name and answer. If you sort only one column, you mix up people and numbers. The sheet then lies and looks fine.

This is the Sorter’s most important warning. Read it slowly.

You have a table:

	A	B
1	Subject	Grade
2	Maths	2
3	Czech	1
4	English	3

If you sort only column B from smallest, the grades jump to 1, 2, 3. The subjects stay. Czech keeps the 1, but Maths gets someone else’s number. Next time the average “fits” — and it sits on the wrong subjects.

⚠️

Do not sort by hand after selecting half the table. Always select the whole table including headings, or use the filter. Otherwise a grade stays with the wrong subject. Sheets sometimes asks “do you want to expand the selection?”. Say yes, expand. Not “sort only the selected cells”.

Turn on the arrows

A filter adds little arrows on the headings. From those you sort and hide. Headings must be in the first row of the table and they must be unique. Two heading rows confuse the filter.

Click anywhere in the table with headings. For example on Znamky in A1. You do not have to select the whole range — the sheet guesses it when there are no holes in the data.
At the top Data → Create a filter (Sheets) or Data → Filter (Excel). Little arrows appear at A1 and B1. If they do not, row 1 is not a heading, or you clicked outside the table.
Arrow at Grade → Sort A to Z (smallest first). Whole rows swap. The heading stays on top. It should look like this: Czech 1, History 1, Maths 2, Science 2, English 3 — or according to your numbers. The subject travels with the grade.
Click the row with English. In column B there must still be the 3 you wrote next to English. If there is a 1, you sorted only one column. Ctrl+Z and again through the filter.
Arrow at Grade → uncheck everything except 1. You see only the ones. The other rows vanished from the screen. Row numbers on the left jump (2, 6…), because 3, 4, 5 are hidden.
Arrow at Grade again → Select all. The table is back. Count the rows. There must be as many as before.

💡

The arrow that is filtering is often filled or has a different icon. That is a good sign: “I am hiding something here”. Before you hand in, turn on Select all. Otherwise the teacher will see only the ones and think you have five subjects.

What happens when you sort half

Try this on purpose on a copy, not on your only data.

On Poznamky copy three rows: Adam 2, Bára 1, Cyril 3. Headings Tên, Grade.
Select only B2:B4. Data → Sort range → smallest first. When Sheets asks about expanding, click Sort these cells only — only now, as a test.
Look. Grades go 1, 2, 3. Têns stayed Adam, Bára, Cyril. Adam suddenly has the 1 that belonged to Bára. This is a broken table.
Ctrl+Z. Now select A1:B4 or turn on the filter and sort from the arrow. Têns travel with grades. Bára has 1 again.

Once you see it with your own eyes. Then you will not click “just that column” again.

A filter is not a delete

A hidden row is alive. The formula =PRŮMĚR(B2:B6) still counts it, even when you cannot see it. A filter is glasses, not an eraser.

If you want the average only from what is visible, you need a different function (SUBTOTAL / SUBTOTAL) — you do not need that yet. For now remember: hidden ≠ deleted. The average under the table may not change after you filter out the ones. That is fine.

Deleting is something else: you select the row, right-click, Delete row. That does not come back by itself. Do not use the filter for that.

What often goes wrong

Sorting one column. Half the table moves, half does not. Ctrl+Z. Then the filter.

The heading slid into the middle. You selected row 1 as data too. The filter can “miễn phíze” the first row. If you sort without a filter and select A1:B6 as values, Subject jumps between Maths. Turn on the filter. The heading stays.

A hole in the middle. Empty row 4. The filter thinks the table ends on row 3. Delete the hole, or select the range by hand.

Two headings. Row 1 Grades March, row 2 Subject / Grade. The arrows sit on March and sort nonsense. Put the table title outside, or in the sheet header. The first row of the data table = column names.

📊 Survey — filter

Headings Tên (or Number, if you do not want names) and Answer. Turn on the filter. Arrow at the answer: leave only one flavour checked, say apple. Count the visible rows. That is the vote count. Once you know KDYŽ, let the table count by itself. For now eyes and a filter are enough. Then Select all, so your data does not “get lost” before you hand in.

Now you 💪

Two rounds on Znamky, one on the survey.

First: turn on the filter, sort grades from smallest, check that the subject travelled with the grade. Filter only the ones. Bring everything back. Not a single row may disappear for good.

Second: on Poznamky repeat the “only column B” test. Watch how names split from numbers. Ctrl+Z. You should remember this more than the arrow.

If you have a survey, filter one answer and count the rows.

✅

Check: Arrows on the headings. You can hide and show again. You sort whole rows. You know what a table looks like after sorting half — and you do not do it.

What to take from this lesson

A filter hides. Sorting shuffles. Neither deletes.
Always the whole row, not one column. Half a table = someone else’s grades.
Headings must be in the first row of the table. Before you hand in, select all.