Zero foundation self-study SQL course | having clause

Hello everyone , I'm Ning Yi .

Today's talk SQL Course No 16 course ：HAVING Clause .

HAVING It is also a conditional filter statement , Put it in GROUP BY Behind .

Basic grammar ：

SELECT  < Field name > 
FROM  < Table name >  
GROUP BY < Field name >
HAVING  < filter >;

1、HAVING And WHERE The difference between

HAVING And what we learned before WHERE Are all conditional filter statements , Not only do they have the same effect , The writing is also similar .

WHERE The comparison operator after the statement 、IN、BETWEEN、LIKE etc. , stay HAVING Can also be used in .

The essential difference between the two is ：

WHERE Is in GROUP BY Filter the conditions before grouping , Aggregate functions cannot be followed .

HAVING Is in GROUP BY Condition filtering after grouping , It can be directly followed by the aggregate function .

example ： stay Students In the table , Find the student number Sid Less than 8 The record of , And find out the number of male and female students each head teacher brings , The last output quantity is greater than 2 The record of .

Sample results ：

Instance analysis ： We want to output more than 2 The record of , The screening criteria are COUNT(*)>2,WHERE It can't be followed by an aggregate function , So this filter condition should be placed in HAVING Behind .

SELECT Tid,Ssex,COUNT(*)
FROM Students
WHERE Sid<8
GROUP BY Tid,Ssex
HAVING COUNT(*) > 2

Some students here may also have questions , since HAVING Can also do conditional screening , Then we can put the top SQL Of the statement WHERE Get rid of , Filter criteria are merged into HAVING Medium is OK , It looks like this ：

SELECT Tid,Ssex,COUNT(*)
FROM Students
GROUP BY Tid,Ssex
HAVING Sid<8 AND COUNT(*) > 2

You can run this statement , Will report a mistake Unknown column 'Sid' in 'having clause’, We can't write like this .

because HAVING If the following field is an existing column in the table , Then this column must appear in SELECT Back .

For example, we will HAVING hinder Sid Change to Tid, The statement is correct , because Tid, Also in the SELECT Behind .

HAVING Tid<8 AND COUNT(*) > 2

2、 Field summary

HAVING Field 、SELECT Field , In addition to the above GROUP BY Field , The relationship between them in the following columns , You may feel a little confused , Let's summarize here as a whole , Straighten out your thinking .

Finally, it can be summarized as the following SQL sentence , among A、B、C Are all existing columns in the table .

SELECT A, C,COUNT(A)
GROUP BY A,B,C
HAVING A>1

GROUP BY If it is A,B,C：

SELECT The following column , As long as it is included in A,B,C Medium will do . It can be A,B,C, It can also be A,B, It can also be A,C, It can also be A, But it can't be A,D.

SELECT If it is A,C：

HAVING The following column , As long as it is included in A,C Medium will do , It can be A,C, It can also be A, It can also be C, But it can't be B.

That is to say GROUP BY Columns that do not appear in ,SELECT Can't appear in ,SELECT Columns that do not appear in ,HAVING Can't appear in .

It's a little twisted , You can feel it according to the above example ~

The reason is that , It is related to the execution order of statements , We will explain the execution sequence of statements in detail in the following courses .

Homework ： combination Students Table and Teachers surface , Find the number of male and female students each head teacher brings , The number of students in each group is greater than 2 The record of .

Sample results ：

Homework analysis ： Answer according to the example , The first column is the name of the head teacher ,Students There are only teacher numbers in the table Tid, So we need JOIN Connect Teachers surface , Get the name of the head teacher .

Also find out the number of male and female students each head teacher brings , adopt GROUP BY To the head teacher Tname, Student gender Ssex grouping , Re pass COUNT(*) Just count the quantity .

Finally, the number of students in each group is greater than 2 The record of , The screening criteria are COUNT(*)>2,WHERE It can't be followed by an aggregate function , So this filter condition should be placed in HAVING Behind .

SELECT
  t.Tname AS " Teacher name ",
  s.Ssex AS " Student gender ",
  COUNT(*) AS " Number "
FROM Teachers AS t
JOIN Students AS s
ON t.Tid = s.Tid
GROUP BY t.Tname,s.Ssex
HAVING COUNT(*)>2;

In the next class, I will explain in detail SQL The order in which statements are written and executed .

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