MYSQL06_ Seven join operations and union all of sql99

• ①. SQL99, grammar ( Recommended )

1. If it's a left outer connection , The table on the left of the join condition is also called the main table , The table on the right is called the slave table
2. If it's a right outer connection , The table on the right in the join condition is also called the master table , The table on the left is called the slave table
3. full join mysql Is not supported , have access to union all Instead of

select  Query list 
	from  surface 1  Alias [ Connection type   It can be omitted ]
	join  surface 2  Alias 
	on  Connection condition 
	[where filter ]
	[group by grouping ]
	[having  filter ]
 Internal connection : [inner] join
 External connection :
 Left lateral ：left[outer] join
 Right outside ：right[outer] join
 Total external ：full[OUTER] join

• ②. The illustration 7 A connection
  
• ③. Merge query results utilize UNION keyword , You can give more than one SELECT sentence , And combine their results into a single result set . When merging , The number of columns and data types of the two tables must be the same , And correspond to each other . each SELECT Use... Between statements UNION or UNION ALL Keyword separation

SELECT column,... FROM table1 
UNION [ALL] 
SELECT column,... FROM table2

• ④. UNION Operator returns the union of the result sets of two queries , Remove duplicate records
  

• ⑤. UNION ALL Operator returns the union of the result sets of two queries . For duplicate parts of two result sets , No weight removal ( Recommended )