你知道mysql哪些查询情况不走索引吗

SELECT `username` FROM `t_user` WHERE age=20;-- 会使用索引
SELECT `username` FROM `t_user` WHERE age+10=30;-- 不会使用索引！！因为所有索引列参与了计算
SELECT `username` FROM `t_user` WHERE age=30-10;-- 会使用索引

-- 不会使用索引,因为使用了函数运算,原理与上面相同
SELECT username FROM t_user WHERE concat(username,'1') = 'admin1'; 
-- 会使用索引
SELECT username FROM t_user WHERE username = concat('admin','1'); 

SELECT * FROM USER WHERE username LIKE 'mysql测试%'   --走索引
SELECT * FROM USER WHERE username LIKE '%mysql测试'   --不走索引
SELECT * FROM USER WHERE username LIKE '%mysql测试%'  --不走索引

-- stock_code字符串类型带索引
SELECT * FROM `stock_data` WHERE stock_code = '600538'  --走索引
SELECT * FROM `stock_data` WHERE stock_code = 600538  --不走索引

-- stock_code带索引，open不带索引
SELECT * FROM `stock_data` WHERE `stock_code` = '600538' OR `open` = 6.62  -- 不走索引
-- stock_code带索引，up_down_pre带索引
SELECT * FROM `stock_data` WHERE `stock_code` = '600538' OR `up_down_pre` = 5.1  -- 走索引

SELECT * FROM t_user WHERE username <> 'mysql测试'

SELECT * FROM t_user WHERE username IS NULL -- 不走索引
SELECT * FROM t_user WHERE username IS NOT NULL -- 不走索引

-- stock_code数据类型为varchar
SELECT * FROM `stock_data` WHERE `stock_code` IN ('600538')  -- 走索引
SELECT * FROM `stock_data` WHERE `stock_code` IN ('600538','688663','688280')  -- 走索引
SELECT * FROM `stock_data` WHERE `stock_code` IN (大量数据)  -- 不走索引
SELECT * FROM `stock_data` WHERE `stock_code` IN (600538)  -- 不走索引

select * from stu where name = ‘111'；走索引
select * from stu where name = 111；不走索引

EXPLAIN SELECT * FROM teltab WHERE telnum LIKE ‘31%' 走索引
EXPLAIN SELECT * FROM teltab WHERE telnum LIKE ‘%110' 不走索引