A simple database return query problem

If the bonus is empty, then your salary plus bonus sal+comm is also empty.

You can use the nvl function nvl( string1, replace with) function: if string1 is null, the nvl function returns the value of replace with, otherwise it returns the value of string1.

select sal+nvl(comm,0) as total from emp;

Wrong. SQL statements cannot be added directly.

The landlord, on the first floor, can basically solve your problem, but the execution efficiency is relatively low, because the two functions are called. In fact, you can just use union directly, because union all will go out of duplicate lines, and the difference between union and union is as follows.

The two rules are as follows.

union, which performs a union operation on two result sets, excluding duplicate rows, and sorts the default rules at the same time;

union all, which performs a union operation on two result sets, including duplicate rows, without sorting;

It should be used. select number from a;

unionselect number from b;

select distinct number from a;

union all

select distinct number from b;

Hope it helps

Suppose the table name is dept,select, from dept d,(select sum(salary),dm from dept group by dept) b where

In this way, we find the proportion of wages in the sector for each ID, and we get a percentage.

It should be this logic, brother, take a look.

declare @tab table(id int,name nvarchar(30),pic int)

insert into @tab

select 1,'a',1000

unionselect 2,'b',1500unionselect 3,'a',1500unionselect 4,'b',1500unionselect 5,'b',1200select id,,pic,pics,convert(float,(pic* from @tab a

left join

select name,sum(pic) as pics from @tab group by name)bon

SQL statement:

select [id],dm,salary,total,convert(nvarchar(10), salary*100)/total)+'%'as pay ratio from table a left join

select [dm] ,sum([salary]) total from table group by dm) b on

table is the table name, total is the total salary of the department, and the others are the data you already have.

Make an entity class and put the read data in the list <> according to the entity. It would be nice to just read "student name" in the loop.

nuique means that this column cannot have duplicate fields and can have null values.

It's pretty much the same as the primary key, but the primary key doesn't allow null values.

distinct is a field that de-duplicates the query set.

Your query statement above should be true, maybe you entered it under the Chinese input method.

select count(distinct teacher) from datasheet.

This can be done on my machine, and it can be executed with the correct result.

You try the statement again.

If there are still errors, please point them out.

Supplement: There are a lot of things that can be installed on XP, such as sql2005 standard edition, sql2000, etc., but I am not very familiar with access.

The first question uses nested queries.

Use the unique constraint to ensure that no duplicate values are entered in non-primary key columns. Although both the unique and primary key constraints enforce uniqueness, you should use the unique constraint instead of the primary key constraint when enforcing the uniqueness below:

A column or combination of columns that are not the primary key.

A table can define multiple unique constraints, but only one primary key constraint.

Columns that allow null values.

A unique constraint can be defined on a column that allows null values, but not a primary key constraint.

select count(distinct (teacher) )from table.

unique is the property of the column itself, which means that the data in it is unique, and distinct is used when querying, if the column has the same data, only the first one will be returned.

select count(teacher) from table awhere !=

nuique should be unique.

distinct to find distinct data.

select count (distinct teacher)

Are you wrong like this? I'm running fine under sql server 2k5.

I'll give you the idea to write it yourself, first you have to start with the fifth, check four pieces of data. In other words, the first nine pieces of data should be removed.

Then the IDs of the first nine queries are not in the top five.

With subqueries, can you understand that?

I don't understand hi me.

From the fifth onwards, it should be written like this.

select * from news where kid=3 order by niddesc limit 4,4;

Change 5 to 4 (note that mysql starts from 0).