SQL Touch a question, SQL elementary syntax questions, high scores

Problem 1 : not exists indicates that when the result set of the subquery is empty, the condition is valid!

For example, if the student table has data s = 100, then the subquery of the matching record is.

select * from sc where = 100 and c#='c2'

If the subquery fails to obtain the result, the data in the parent query S table s = 100 satisfies one of the data of the SQL statement.

Question 2 : If it is <> the explanation in this query is that if only one student has learned, this student will be displayed, and if none of them have learned, all of them will be displayed.

Basic grammar.

create trigger tri_name

on table

for insert/update/deleted

assql code

Example: Create trigger tri mc (tri mc: the name of the custom trigger).

on table (table: the name of the table that was triggered).

for insert (trigger type: insert, update triggered by inserting data, delete triggered by updating data, and triggered by deleting data).

asdeclare @kssj datetime (define a variable and do intermediate processing).

declare @sqbh char(40)

select @kssj = kssj,@sqbh = sqbh from inserted

When data is inserted, the inserted data is inserted in the table

When the data is updated, the updated data is deleted in the table after the table is inserted The data before the update is deleted

When you delete data, the deleted data is deleted in the table

Different operations can take out different intermediate data and do processing, in the example to take out the inserted data).

update table2 set isks = 1,kksj = @kssj where sqbh = @sqbh

Do the corresponding operation according to the intermediate processing, in the case of the processing of the insertion, and update the inserted value to another table synchronously).

Take a closer look at this paragraph**:

update borrowrecord

setfrom borrowrecord br , deleted d ,inserted i

wherefrom borrowrecord br , deleted d ,inserted i

It means that the alias br is used instead of borrowrecord, d is used instead of deleted, and d is used instead of inserted.

Got it? good luck!

if (object_id('tgr_classes_update_column', 'tr') is not null)

drop trigger tgr_classes_update_column

gocreate trigger tgr_classes_update_column

on student

for update

as--column-level trigger: whether the ID field of student has been updated.

if (update(id))

beginupdate score set id=** where ..Modify the id=** of the score table

rollback tran;

endgo

Use triggers, search the Internet for trigger usage.

Do you have this sqlserver or oracle or mysql?

Two ways.

1. Triggers.

2. Stored procedures.

Personally, I feel that the storage process is better.

1 is a constant. Here it just represents existence.

If there is no data in the inserted or deleted table, select will not produce data.

If there is data in the table, select displays 1

Writing 1 here has the same effect as *, exists only determines whether there is data in auspicious and subdued, and will not output the bridge width data.

It is best to add an id field to table a, and then if you want to say that the first row id is 1, the second row is 2 and then if you want Chinese units such as meters and kilograms, you just need to add the condition where when taking table a'2'It should be fine.

Try it, in fact, I'm new to SQL, hehe.

A and B don't matter much, and you don't need A.

Select unit = case u001 when'00' then 'm' when '01' then 'kg' when '02' then 'grams'end,* from b) Actually, I think the method on the second floor is good, you can don't need table a, set it directly.

A and B don't matter much, and you don't need A.

Select unit = case u001 when'00' then 'm' when '01' then 'kg' when '02' then 'grams' end,* from b