Which of the following mysql statements does it mean?

What does mysql statement mean? SELECTtid, MAX (a. ctime) ASa_time // What is the use of max? A. What does ctime mean? FROMtcASa // a indicates the table currently queried? LEFTJOINtutorASbONa. tid = B. mysql statement. who can help me see what it means?
SELECT tid, MAX (a. ctime) AS a_time // What is the use of max? A. What does ctime mean?
FROM tc AS a // a indicates the currently queried table?
Left join tutor AS B ON a. tid = B. id // B indicates the tc table? What does this sentence mean?
WHERE a. ctime> 1340590489
AND time <> 0
Group by tid
Order by a_time DESC
LIMIT 20

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A indicates the tc table, B indicates the tutor table, and max indicates the maximum value in the ctime obtained from the tc table.
------ Solution --------------------
The entire statement is left join query.
FROM tc AS a // a indicates the currently queried table?
A is the alias of tc.
// What is the use of max? A. What does ctime mean?
Max is a mysql function.
A. ctime is the ctime field of a (tc table ).
Left join tutor AS B ON a. tid = B. id // B indicates the tc table? What does this sentence mean?
B is the alias of the tutor table. on is followed by the connection condition.

This is the foundation of mysql. I suggest you go to the manual.