相关疑难解决方法(0)

我有以下查询:

SELECT t.id
FROM account_transaction t
JOIN transaction_code tc ON t.transaction_code_id = tc.id
JOIN account a ON t.account_number = a.account_number
GROUP BY tc.id

当我做EXPLAIN第一行时,除其他外,显示:

table: t
type: ALL
possible_keys: account_id,transaction_code_id,account_transaction_transaction_code_id,account_transaction_account_number
key: NULL
rows: 465663

为什么键为NULL？