Workaround for sorting and filtering of merged cells

First step: Unmerging all merged cells

   1.   Select all cells in the worksheet.

A quick way to do so is to click the triangle at the intersection of the row headers and column headers:

   2.   On the Home tab, in the Alignment group, click Merge & Center:

When you click this button, all selected cells in the worksheet will be unmerged.

Note: If the Merge & Center button isn't highlighted, there are no merged cells in the selection.

Second step: Filling the Gaps in a Sheet

   3.   Select the range that has the gaps.

   4.   On the Home tab, in the Editing group, choose the Find & Select drop-down list and then click Go To Special...:

   5.   In the Go To Special dialog box, select the Blanks option and click OK:

This action selects the blank cells in the original selection.

   6.   On the Formula bar, type an equal sign (=) followed by the address of the first cell with an entry in the column. For this example, = B5:

   7.   Press Ctrl+Enter:

Third step: Remove the formulas

   8.   Reselect the original range and press Ctrl+C to copy the selection. In this example, cells B5:B12.

   9.   On the Home tab, in the Clipboard group, choose the Paste drop-down list and then click Values (V) to convert the formulas to values:

After completing these steps, the gaps are filled in with the correct information. Now it's a regular list, and you can do whatever you like with it - including sorting, filtering, etc.

See also this tip in French: Comment trier et filtrer des cellules fusionnées.