Preface:
1，in 和 exists
2，not in 和 not exists
3, in and =
text:
1，in 和 exists
It is in the table for the inner and outer hash (dictionary sets) connection
exists is look for cycles each inner table and then query
Query efficiency:
It has been considered to be inaccurate than exists in high efficiency argument, if the query two tables of comparable size, and exists in it with little difference;
If the two tables in a small a large, large look-up table of the child with exists, a small table with a subquery in
- Table A (small table), Table B (large table) - low efficiency, and uses a table index on the ID column A SELECT * from A WHERE a.id in ( SELECT b.id from B); - efficiency high index uses the ID column of table B SELECT * from a WHERE EXISTS ( SELECT b.id from B WHERE b.id = a.id) - high efficiency, and uses a table index on the ID column B SELECT * from B WHERE b.id in ( SELECT a.id from A) -Inefficient, uses the index table id column A SELECT * from B WHERE EXISTS ( SELECT a.id from A WHERE a.id = b.id)
2，not in 和 not exists
not in, if any sub-query returns a record containing null values, the query will not return any records. If there is non-empty sub-query field restrictions, you can use.
Query efficiency:
not in appearance inside will have a full table scan, did not use the index; and the child still can not exists query to the index on the table.
So no matter which table is large, not exists than not in fast.
3, in and =
- The following two are equivalent sql SELECT name from Employee WHERE name in ( ' John Doe ' , ' John Doe ' , ' Wang Wu ' ); SELECT name from Employee WHERE name = " John Doe " or name = ' LI four ' or name = ' Wang Wu ' ;
Reference blog:
SQL query in, exists, not in, not exists with the use of database technology _Linux difference _ commune -Linux system portal
https://www.linuxidc.com/Linux/2016-04/130285.htm