HeidiSQL is one of the nicest SQL GUIs I have used, and it happens to also be free. The drawback for us Linux users is that it is a Windows app, but it works fairly well in WINE if you change the default fonts (which look horrible).

An issue with running HeidiSQL cropped up in versions after 9.5.0.5203. Release 5203 worked in WINE but ones after it would immediately crash when started. A work-around was found by a fellow named Vasilis Lourdas and was posted on the Github bug tracker. The work-around is as follows:

1. run winecfg (launch it from a terminal)
2. click on the Libraries tab along the top
3. click on the drop-down menu under “New override for library” and choose d2d1
4. click Add
5. Highlight d2d1 in the lower list where it now appears
6. click on the Edit button
7. select Native (Windows) and click OK

Now HeidiSQL release after 5203 will work.

The 2020 releases running in Wine 5.0 were giving me trouble: they would have issues resizing the window and would crash frequently. I have typically downloaded the EXE file and run it from the command line—e.g., wine heidisql64.r6013.exe—because the Windows installer would often fail to run in older versions of WINE. I tried again to run the Windows installer with build 6013 and it completed without a problem, and since then it has run reliably.

One annoying issue remains, however: When using backspace key while editing a field it jumps to the Host tab, which is a bit alarming, but you can go back to the tab where you were and resume editing. To edit a field I need to use the back-arrow and the DELETE key. This may be a new WINE default, since I have an older computer that I have updated to WINE 5.0 and it has never had this issue, but multiple new installations of WINE have all had this issue. The suggestions in the HeidiSQL help forum are no help, unfortunately.

Here are some very brief reviews of other Linux SQL GUI tools I have tried.

1. Valentina Studio — a close runner-up to HeidiSQL except when it comes to modifying a table's schema or creating new tables. HeidiSQL is much more intuitive and doesn't hide things (like field comments) behind a tiny pencil icon. Another minor annoyance with VStudio is that it will only reformat SQL code to be more readable once; after that it asks you to buy the paid version.
2. dBeaver seemed decent at first, although it initially is slow to load all of its Eclipse libraries. However, I ran into a mysterious problem where a change to a database and does not replicate to slaves. I spent quite a while trying to figure out what was going on, and eventually found other people reporting the same issue with dBeaver. In a forum I read an experience like mine, but the dBeaver devs figured it was the user's fault and not that of their software. They gave some suggestions, all of which I tried without success, so I added my comments to the forum and then gave up on this package and would only suggest trying it if you are not using replication.
3. MySQL Workbench has got to be the worst of the lot, especially if you are using MariaDB. It seems okay at first, even though it will tell you that MariaDB is not supported, but as you try to use it you will see strange information that you know is wrong and get pop-up messages like “too many open files”.
4. JPDB Admin is a product made for MariaDB from a publisher in Brazil. It seems decent at first, but there are a lot of features and capabilities that are missing or inconvenient to use, so I gave up on this one, too.