Brush question database
1. After the connecting operation and a plus and plus conditions where distinction
2.having used in the back group, you can use a function of previous values aggregate to make a judgment. and having a group by supporting the use of. Conditional inquiries packet
having a filter group
where the former packet filtering can not be behind in the group by
having a value after the search filter
where is the value before the lookup filter
3. How to create a table, to add a field constraints, plus the unique, write in the construction index table
4. unique index: a table can have multiple indexes
general indexes: the only difference between the index and when it encounters data matching will always find, until it does not, the only the index found for not looking.
Primary key index: consistent with the physical order of logical order.
Joint index: an index composed of multiple fields.
Clustered index and non-clustered index: leaf nodes store data aggregation index. Non-clustered index data is not stored
cover index: leaf node stores all data.
After 5.innodb and myisam restart If auto_increasing, after innodb will restart if you delete will disappear.
myisam not disappear.
6.TRUNCATE TABLE "table name"; compared to the drop table, this statement removes only the data, not the entire table are deleted.
7. isolation level of the database.
Read uncommitted, Read committed, Repeatable read and Serializable
8. how the database index
create index index name on table name (c1, c2)
the Create UNIQUE index index name on table name (c1, c2 ..)