Left connection. The right connections and all connections

2010-10-12  来源:本站原创  分类:Database  人气:128 

Connect the left and right connections and all connections

Null value (NULL) some field values, null values may occur, because the data do not know what value or does not exist.

Empty string value is not equivalent to the space, nor is it numeric 0. Therefore, to determine whether the value of a field can not be used when a null value =, ">" Those judgments character. Must have specific phrases: IS NULL to select the value field of the records available, empathy, IS NOT NULL can select the records do not contain null values.

A left join (left join) the left side of the table contains all the records and even the right of the table does not match its records.