join, if the field is null

The result of the connection should not include a tuple with an empty value of field, so it will not be wrong, but it will not be selected;

Keywords are not allowed to be empty, and they are unique, so there will be no 1, and query efficiency may be affected, except that there should be no difference.

1. When there is a null value, the join will not be wrong, but the associated data will not be obtained, because the null value is not compared with any value.

2. Whether field is a keyword or not will have a certain impact on the speed of the number of queries It's hard to say.

on is followed by the join conditions of the two tables, and if they do not match, the right table is null

Two columns A B from Table1, and two columns B and C from Table2.

select t1.*,case when is null then 0 else end) as c 。

from table1 t1 left join table2 t2 on(。

One. Basic concepts.

null is a special metric value (or an object reference) that indicates that the indicator does not point to any object.

Two. nullvalue

In many definitions, null can mean three-valued logic, with null"no value"Or"unknown value"。

The SQL database query language uses Null in this place, as in Visual Basic and its derivatives. On this model, null operand is equivalent to null (vb) or"unknown" (sql)。

So for a, the equation"a == null"with"a <>null"Neither true nor false. However, the Bollinger operation"a and false"Produces false, and something like that"a or true"is true even when a is null.

This is because the equation does not represent a domain. Another equation"a is null"with"a is not null"Both are the right ways to test the null value in SQL, and some SQL can be implemented as a == null can be"true"if a is null; See null (sql) for details.

The result set is an object, and if there is no record, it is empty, not 0

update tb t1

inner join (select a,max(b) from tb group by a where is not null) t2 on

setwhere is null

Theoretically, it's sauce purple. Be careful to back up your data when testing!

update table set b=case when a='Little Red' then '100' when a='Xiaolan' then '300' end

where a in('Little Red','Xiaolan')

Just use full join.

select ,,isnull(,'Specify a string') from Table 1 a full join Table 2 b on

If it's sql server 2k, then use isnull(name, 'none') as name in the select statement, and if it's oracle, then use nvl(name, 'none') as name, is that what it means?

You might as well just use the insert statement.

insert into a(name) values('Not yet')

select ,isnull(,'For the time being, I will return to Jingliang without missing the manuscript') as name, from #a left join #b on

You can add the field name to the query and then escape the field name.

For example: select as id, as name1, as name2 from table1 as t1 left join table2 as t2 where =

In this way, the name of the queried field is the name after as, and there will be no overlap of name values.