Join syntax (that is connected to several tables into a virtual table, easy access)
Cross-connect
Indeed, cross-connection is connected to the two tables do not set any condition results.
Cross-connection is also commonly called a "Cartesian product" - may be more mathematically.
from 表1 join 表2
More invalid data is not recommended
En:
Table 1 from the Join Table 1. Table 2 ON = Table 2. Field 1 Field 2;
Find (filtered) value field in Table 11 is the results of the cross-connect table is equal to those of Table 2 Field 2 of row values.
The most widely used,
A plurality of tables connected
SELECT * FROM Table. 1 JOIN Table 2 ON = Table 2. Table 1 Field Field
JOIN Table 2. Table 3 ON = Table 3 Field Field
Left [outer] connection
from Table 1 left [outer] join in Table 2 on the connection conditions.
In the left side of the main table table, empty columns are automatically filled with null
Right [outer] Connection:
from Table 1 right [outer] join in Table 2 on the connection conditions.
In the table on the right side of the main table, empty columns are automatically filled with null
Subqueries to find a common point connected in series
In can be connected to a plurality of values
where in the operand (value 1, value 2, ....)
Indicates that the operand (field value) is equal to the subqueries only either one, even if the condition
Joint inquiry
Keyword union query is: union
Union query is to query the results of two select statements of "stacked" together to become a "great result."
Query results can be two "joint" of pre-conditions are: the result is equal to the number of fields. Column is equal
1 select statement
union
select statement 2;
1, the output section of two select statements (result fields) the same number, the same type of application typically makes sense.
2, the result set of a select field is subject to the first sentence of the field.
3, a first field select statement can do the alias, the alias if done, subsequent where, group, order, etc. should be used the alias clause.
4, if you want to sort or limit the entire joint results, you should select for their statements in parentheses:
(Select statement 1)
union
(Select statement 2)
order by ..... limit ....；