Database notes

1、 Case sensitivity of attribute values in the same column

    When three records b_author Property values are set to  ab,AB,Ab, Use 

delet from book where b_author=‘AB’

   Three records in the table have been deleted .

reason ：

  mysql Queries are case insensitive ！ This really shocked me , Although I know that generally , Keywords are case insensitive , But I didn't expect that even the parameters to be queried are case insensitive ！！ Try the following sql sentence , Sure enough, the result is the same .

  Mysql Default character retrieval strategy ：utf8_general_ci, Indicates case insensitive ;utf8_general_cs Means case sensitive ,utf8_bin Represents a binary comparison , Also case sensitive  .（ Be careful ： stay Mysql5.6.10 In the version , I won't support it utf8_genral_cs！！！！）

   Create table time , Set the table directly collate The attribute is utf8_general_cs perhaps utf8_bin; If you have created a table , Then directly modify the Collation The attribute is utf8_general_cs perhaps utf8_bin.

2、MySQL Of “utf8mb4” It's true. “UTF-8”.MySQL Of “utf8” It's a kind of “ Exclusive code ”, It can code Unicode There are not many characters .

Be careful ,utf8mb4 And utf8 One difference is that the former can store ink expressions .

mysql The coding utf-8 and utf8mb4 difference

3、between and It contains two boundaries .