Today, I spent a long time to find a bug. I read code, but finally I found the cause is that the character set of table is not correct.

Record this to avoid making the same mistake. Character set should always be set while a table is created. For MySQL, the syntax is:

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CREATE TABLE tbl_name (column_list)
    [[DEFAULT] CHARACTER SET charset_name]
    [COLLATE collation_name]]

ALTER TABLE tbl_name
    [[DEFAULT] CHARACTER SET charset_name]
    [COLLATE collation_name]

Example:

CREATE TABLE t1 ( ... )
CHARACTER SET utf8;

Sometimes, you need to see what character set a database/table/column is, here is the solution I found at stackoverflow:

For Schemas:

SELECT default_character_set_name FROM information_schema.SCHEMATA S
WHERE schema_name = "schemaname";

For Tables:

SELECT CCSA.character_set_name FROM information_schema.`TABLES` T,
       information_schema.`COLLATION_CHARACTER_SET_APPLICABILITY` CCSA
WHERE CCSA.collation_name = T.table_collation
  AND T.table_schema = "schemaname"
  AND T.table_name = "tablename";

For Columns:

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SELECT character_set_name FROM information_schema.`COLUMNS` C
WHERE table_schema = "schemaname"
  AND table_name = "tablename"
  AND column_name = "columnname";