From my guide How to support full Unicode in MySQL databases, here are the queries you can run to update the charset and collation of a database, a table, or a column:
For each database:
ALTER DATABASE
database_name
CHARACTER SET = utf8mb4
COLLATE = utf8mb4_unicode_ci;
For each table:
ALTER TABLE
table_name
CONVERT TO CHARACTER SET utf8mb4
COLLATE utf8mb4_unicode_ci;
For each column:
ALTER TABLE
table_name
CHANGE column_name column_name
VARCHAR(191)
CHARACTER SET utf8mb4
COLLATE utf8mb4_unicode_ci;
(Don’t blindly copy-paste this! The exact statement depends on the column type, maximum length, and other properties. The above line is just an example for a VARCHAR column.)
Note, however, that you cannot fully automate the conversion from utf8 to utf8mb4. As described in step 4 of the abovementioned guide, you’ll need to check the maximum length of columns and index keys, as the number you specify has a different meaning when utf8mb4 is used instead of utf8.
Section 10.1.11 of the MySQL 5.5 Reference Manual has some more information on this.
