I tested several simple queries like such:

SELECT COUNT(*) FROM Users WHERE courses='a';

But none are being cached. The following are the cache related variables:

show variables like "%cache%";
+-------------------------------+----------------------+
| Variable_name                 | Value                |
+-------------------------------+----------------------+
| aria_pagecache_age_threshold  | 300                  |
| aria_pagecache_buffer_size    | 100000000            |
| aria_pagecache_division_limit | 100                  |
| binlog_cache_size             | 32768                |
| binlog_stmt_cache_size        | 32768                |
| have_query_cache              | YES                  |
| join_cache_level              | 2                    |
| key_cache_age_threshold       | 300                  |
| key_cache_block_size          | 1024                 |
| key_cache_division_limit      | 100                  |
| key_cache_segments            | 0                    |
| max_binlog_cache_size         | 18000000000000000000 |
| max_binlog_stmt_cache_size    | 18000000000000000000 |
| metadata_locks_cache_size     | 1024                 |
| query_cache_limit             | 1048576              |
| query_cache_min_res_unit      | 4096                 |
| query_cache_size              | 0                    |
| query_cache_strip_comments    | OFF                  |
| query_cache_type              | ON                   |
| query_cache_wlock_invalidate  | OFF                  |
| stored_program_cache          | 256                  |
| table_definition_cache        | 400                  |
| table_open_cache              | 400                  |
| thread_cache_size             | 0                    |
+-------------------------------+----------------------+

The cache related status shows:

show status like "%cache%";
+-----------------------------------+-------+
| Variable_name                     | Value |
+-----------------------------------+-------+
| Aria_pagecache_blocks_not_flushed | 0     |
| Aria_pagecache_blocks_unused      | 15737 |
| Aria_pagecache_blocks_used        | 2     |
| Aria_pagecache_read_requests      | 78    |
| Aria_pagecache_reads              | 3     |
| Aria_pagecache_write_requests     | 6     |
| Aria_pagecache_writes             | 0     |
| Binlog_cache_disk_use             | 0     |
| Binlog_cache_use                  | 0     |
| Binlog_stmt_cache_disk_use        | 0     |
| Binlog_stmt_cache_use             | 0     |
| Com_assign_to_keycache            | 0     |
| Qcache_free_blocks                | 0     |
| Qcache_free_memory                | 0     |
| Qcache_hits                       | 0     |
| Qcache_inserts                    | 0     |
| Qcache_lowmem_prunes              | 0     |
| Qcache_not_cached                 | 0     |
| Qcache_queries_in_cache           | 0     |
| Qcache_total_blocks               | 0     |
| Ssl_callback_cache_hits           | 0     |
| Ssl_session_cache_hits            | 0     |
| Ssl_session_cache_misses          | 0     |
| Ssl_session_cache_mode            | NONE  |
| Ssl_session_cache_overflows       | 0     |
| Ssl_session_cache_size            | 0     |
| Ssl_session_cache_timeouts        | 0     |
| Ssl_used_session_cache_entries    | 0     |
| Subquery_cache_hit                | 0     |
| Subquery_cache_miss               | 0     |
| Threads_cached                    | 0     |
+-----------------------------------+-------+

I have read through the manual and checked that the following conditions are satisfied:

• the query_cache_type server variable is set to 1, or ON
• the following functions are not present in the query: BENCHMARK()
  CONNECTION_ID() CONVERT_TZ() CURDATE() CURRENT_DATE() CURRENT_TIME()
  CURRENT_TIMESTAMP() CURTIME() DATABASE() ENCRYPT() (one parameter)
  FOUND_ROWS() GET_LOCK() LAST_INSERT_ID() LOAD_FILE()
  MASTER_POS_WAIT() NOW() RAND() RELEASE_LOCK() SLEEP() SYSDATE()
  UNIX_TIMESTAMP() (no parameters) USER() UUID() UUID_SHORT()

• it is not of the form:

  SELECT SQL_NO_CACHE …
  SELECT … INTO OUTFILE …
  SELECT … INTO DUMPFILE …
  SELECT … FOR UPDATE
  SELECT * FROM … WHERE autoincrement_column IS NULL
  SELECT … LOCK IN SHARE MODE

• it uses a non-TEMPORARY table
• it does not generate a warning
• it does not access a partitioned table
• it does not access a table from INFORMATION_SCHEMA, mysql or the
  performance_schema database
• it does not make use of user or local variables
• it does not make use of stored functions
• it does not make use of user-defined functions
• it is not inside a transaction with the SERIALIZABLE isolation level

The only thing I am not sure is:

• the user does not have a column-level privilege on any table in the
  query

I run SHOW FULL COLUMNS FROM Users; and all columns contain "select, insert, update". Even newly created tables contain those privileges. Can anyone explain what could be wrong?