Here is the command I use:

```
ALTER TABLE <table_name> MODIFY <column_name> VARCHAR(255) CHARACTER SET utf8 COLLATE utf8_unicode_ci;

```

It works well. Now I need to set `utf8mb4_unicode_ci` for a column *(since currently characters are shown as `???`)*. Anyway here is my new command:

```
ALTER TABLE <table_name> MODIFY <column_name> VARCHAR(255) CHARACTER SET utf8 COLLATE utf8mb4_unicode_ci;

```

But sadly MySQL throws:

> 
> ERROacR 1253 (42000): COLLATION 'utf8mb4\_unicode\_ci' is not valid for CHARACTER
> 
> 
> 

Any idea?