100 important knowledge points that SQL must master: Combined Query

14.1 Combination query
Most of the SQL A query contains only a single bar of data returned from one or more tables SELECT sentence .
however ,SQL It also allows multiple queries to be executed （ multiple SELECT sentence ）, And take the result as a
Query result sets return . These composite queries are often referred to as and （union） Or composite query
（compound query）.
There are two main situations where you need to use composite queries ：
   Return structural data from different tables in a query ;
   Perform multiple queries on a table , Press a query to return data .

Tips ： Combine queries and multiple WHERE Conditions
Most of the time , Combining two queries with the same table does the same job as having multiple WHERE
A query with Clause conditions does the same thing . let me put it another way , Any with multiple
WHERE Clause SELECT Statements can be used as a combined query , Below you can see
To this point .

14.2 Create a composite query
You can use UNION Operator to combine several SQL Inquire about . utilize UNION , More than one... Can be given
SELECT sentence , Combine their results into a result set .
14.2.1 Use UNION
Use UNION It's simple , All you have to do is give each SELECT sentence , In each statement
Put keywords between UNION .
for instance , If necessary Illinois、Indiana and Michigan Wait for all the customers in several states in the United States
Customer's statement , Also want to include all... Regardless of the state Fun4All . Of course you can use
WHERE Clause to do this , But this time we use UNION .
As mentioned above , establish UNION Involves writing multiple SELECT sentence . First, let's look at a single statement ：
Input ▼
SELECT cust_name, cust_contact, cust_email
FROM Customers
WHERE cust_state IN ('IL','IN','MI');
Output ▼
cust_name cust_contact cust_email
----------- ------------- ------------
Village Toys John Smith [email protected]
Fun4All Jim Jones [email protected]
The Toy Store Kim Howard NULL
Input ▼
SELECT cust_name, cust_contact, cust_email
FROM Customers
WHERE cust_name = 'Fun4All';

Output ▼
cust_name cust_contact cust_email
----------- ------------- ------------
Fun4All Jim Jones [email protected]
Fun4All Denise L. Stephens [email protected]
analysis ▼
Article 1 with a SELECT hold Illinois、Indiana、Michigan The abbreviations of other states are passed to IN Clause ,
Retrieve all the rows in these States . Second SELECT Use a simple equality test to find all
Fun4All . You will find a record in two results , Because it satisfies twice
Conditions .
Combine these two statements , It can be done as follows ：
Input ▼
SELECT cust_name, cust_contact, cust_email
FROM Customers
WHERE cust_state IN ('IL','IN','MI')
UNION
SELECT cust_name, cust_contact, cust_email
FROM Customers
WHERE cust_name = 'Fun4All';
Output ▼
cust_name cust_contact cust_email
----------- ----------- ----------------
Fun4All Denise L. Stephens [email protected]
Fun4All Jim Jones [email protected]
Village Toys John Smith [email protected]
The Toy Store Kim Howard NULL
analysis ▼
This statement consists of the first two SELECT Sentence composition , In between UNION Keyword separation .

UNION instructions DBMS Implement these two articles SELECT sentence , And combine the output into a query
Result set .
For ease of reference , Here are some examples of using WHERE Clause instead of UNION The same query ：
Input ▼
SELECT cust_name, cust_contact, cust_email
FROM Customers
WHERE cust_state IN ('IL','IN','MI')
UNION
SELECT cust_name, cust_contact, cust_email
FROM Customers
WHERE cust_name = 'Fun4All';
In this simple example , Use UNION Maybe it's better than using WHERE The clause is more complex . but
For more complex filtering conditions , Or from multiple tables （ Not a watch ） To retrieve data from
situation , Use UNION Make it easier to deal with .
Tips ： UNION The limitation of
Use UNION Combine SELECT Number of statements ,SQL There are no standard restrictions . however , most
OK, let's refer to the specific DBMS file , Find out if it's right UNION The largest that can be combined
The number of statements is limited .
Be careful ： Performance issues
Most good DBMS Use internal query optimizer , Dealing with various items SELECT Statement before
Combine them . Theoretically speaking , This means using multiple... From a performance point of view WHERE Clause condition
still UNION There should be no practical difference . But I'm talking about theory , In practice, most
The query optimizer is not in the ideal state , So it's best to test these two methods ,
See which job is better .

14.2.2  UNION The rules
You can see , UNION Very easy to use , But there are several rules to note when combining .
 UNION There must be two or more SELECT Sentence composition , Use keywords between statements
UNION Separate （ therefore , If you combine four SELECT sentence , Three... Will be used UNION
keyword ）.
 UNION Each query in must contain the same columns 、 Expression or aggregate function （ however ,
The columns do not need to be listed in the same order ）.
   Column data types must be compatible with ： The type doesn't have to be exactly the same , But it has to be DBMS Can imply
Type of conversion （ for example , Different numeric types or different date types ）.
explain ：UNION Column name of
If combined UNION The use of SELECT Statement encountered different column names , What will be returned
What's the name ？ for instance , If a statement is SELECT prod_name , And the other one
The sentence is SELECT productname , So what is the name returned by the query result ？
The answer is that it returns the first name , This example will return prod_name , and
No matter the second different name . This also means that you can use an alias for the first name ,
So return a name you want .
This behavior has an interesting side effect . Because only the first name is used , So think
You can only use this name to sort . Take our example , It can be used ORDER BY
prod_name Sort the results , If written ORDER BY productname Will go wrong ,
Because the query result is not called productname The column of .
If these basic rules or restrictions are observed , Then you can put UNION For any data retrieval
operation .

14.2.3 Include or cancel duplicate lines
go back to 14.2.1 section , Let's look at the SELECT sentence . Notice that the statements are executed separately
when , Article 1 with a SELECT Statement returns 3 That's ok , Second SELECT Statement returns 2 That's ok . And in the
use UNION Combine two SELECT After the statement , Only return 4 Yes, but not. 5 That's ok .
UNION Duplicate rows are automatically removed from the query result set ; let me put it another way , Its behavior is related to one
strip SELECT Use more than one in a statement WHERE The clauses and conditions are the same . because Indiana The state has a
Fun4All Company , So two SELECT Statements return the row . Use UNION when , repeat
The row of is automatically cancelled .
This is a UNION Default behavior of , You can change it if you like . in fact , If you want to go back
All matching lines , You can use UNION ALL instead of UNION .
Please see the following example ：
Input ▼
SELECT cust_name, cust_contact, cust_email
FROM Customers
WHERE cust_state IN ('IL','IN','MI')
UNION ALL
SELECT cust_name, cust_contact, cust_email
FROM Customers
WHERE cust_name = 'Fun4All';
Output ▼
cust_name cust_contact cust_email
----------- ------------- ------------
Village Toys John Smith [email protected]
Fun4All Jim Jones [email protected]
The Toy Store Kim Howard NULL
Fun4All Jim Jones [email protected]
Fun4All Denise L. Stephens [email protected]

analysis ▼
Use UNION ALL ,DBMS Do not cancel duplicate lines . therefore , Return here 5 That's ok , among
A line appears twice .
Tips ： UNION And WHERE
At the beginning of this lesson, we said , UNION Almost always complete with multiple WHERE On the same terms
The job of . UNION ALL by UNION A form of , It's done WHERE Clause complete
Can't work . If you really need all the matching lines of each condition to appear （ Include repeating lines ）,
Must use UNION ALL , instead of WHERE .
14.2.4 Sort the combined query results
SELECT The output of the statement is ORDER BY Clause ordering . In use UNION When combining queries , only
Can use one ORDER BY Clause , It must be on the last SELECT After statement . Yes
Apply to result set , There is no way to sort parts of , And sort another... In another way
Part of the situation , Therefore, it is not allowed to use more than one ORDER BY Clause .
The following example is for the previous UNION Sort the returned results ：
Input ▼
SELECT cust_name, cust_contact, cust_email
FROM Customers
WHERE cust_state IN ('IL','IN','MI')
UNION
SELECT cust_name, cust_contact, cust_email
FROM Customers
WHERE cust_name = 'Fun4All'
ORDER BY cust_name, cust_contact;

Output ▼
cust_name cust_contact cust_email
----------- ------------- -------------
Fun4All Denise L. Stephens [email protected]
Fun4All Jim Jones [email protected]
The Toy Store Kim Howard NULL
Village Toys John Smith [email protected]
analysis ▼
This article UNION In the last one SELECT After the statement ORDER BY Clause . although ORDER
BY The clause seems to be just the last one SELECT Components of the statement , But actually DBMS take
Use it to sort all SELECT All results returned by the statement .
explain ： Other types UNION
some DBMS It also supports two other UNION ： EXCEPT （ Sometimes called MINUS ） You can use
To retrieve rows that exist only in the first table but do not exist in the second table ; and INTERSECT
Can be used to retrieve rows that exist in both tables . actually , these UNION Rarely used , because
For the same result, we can get .
Tips ： Manipulate multiple tables
For simplicity , The examples in this lesson are all used UNION To combine multiple queries for the same table
Inquiry . actually , UNION It is also useful when you need to combine data from multiple tables , Even if there is
Tables that do not match column names , under these circumstances , Can be UNION Combined with alias , retrieval
A result set .