# **Derived Tables**

## **Grammar Description**

When independent subqueries are used in the `FROM` clause of the `SELECT` statement, we often refer to it as derived tables, because external queries actually treat the results of the subquery as a data source.

## **Grammar Structure**

Each FROM subquery table must have a name, so the [AS] operator is required. Each column in the SELECT list of the subquery must also have a unique name.

```sql
SELECT ... FROM (subquery) [AS] name ...
```

## **Example**

```text
> CREATE TABLE tb1 (c1 INT, c2 CHAR(5), c3 FLOAT);
> INSERT INTO tb1 VALUES (1, '1', 1.0);
> INSERT INTO tb1 VALUES (2, '2', 2.0);
> INSERT INTO tb1 VALUES (3, '3', 3.0);
> select * from tb1;
+------+---------+
| c1 | c2 | c3 |
+------+---------+
| 1 | 1 | 1.0000 |
| 2 | 2 | 2.0000 |
| 3 | 3 | 3.0000 |
+------+---------+
3 rows in set (0.03 sec)

> SELECT sc1, sc2, sc3 FROM (SELECT c1 AS sc1, c2 AS sc2, c3*3 AS sc3 FROM tb1) AS sb WHERE sc1 > 1;
+------+---------+
| sc1 | sc2 | sc3 |
+------+---------+
| 2 | 2 | 6.0000 |
| 3 | 3 | 9.0000 |
+------+---------+
2 rows in set (0.02 sec)
```````` sql
CREATE TABLE tb1 (c1 INT, c2 CHAR(5), c3 FLOAT);
INSERT INTO tb1 VALUES (1, '1', 1.0);
INSERT INTO tb1 VALUES (2, '2', 2.0);
INSERT INTO tb1 VALUES (3, '3', 3.0);

mysql> select * from tb1;
+------+---------+
| c1 | c2 | c3 |
+------+---------+
| 1 | 1 | 1.0000 |
| 2 | 2 | 2.0000 |
| 3 | 3 | 3.0000 |
+------+---------+
3 rows in set (0.03 sec)

mysql> SELECT sc1, sc2, sc3 FROM (SELECT c1 AS sc1, c2 AS sc2, c3*3 AS sc3 FROM tb1) AS sb WHERE sc1 > 1;
+------+---------+
| sc1 | sc2 | sc3 |
+------+---------+
| 2 | 2 | 6.0000 |
| 3 | 3 | 9.0000 |
+------+---------+
2 rows in set (0.02 sec)
```

-**Subquery with Join**:

```text
> create table t1 (libname1 varchar(21) not null primary key, city varchar(20));
> create table t2 (isbn2 varchar(21) not null primary key, author varchar(20), title varchar(60));
> create table t3 (isbn3 varchar(21) not null, libname3 varchar(21) not null, quantity int);
> insert into t2 values ('001','Daffy','Aducklife');
> insert into t2 values ('002','Bugs','Arabbitlife');
> insert into t2 values ('003','Cowboy','Lifeontherange');
> insert into t2 values ('000','Anonymous','Wannabuythisbook?');
> insert into t2 values ('004','BestSeller','OneHeckuvabook');
> insert into t2 values ('005','EveryoneBuys','Thisverybook');
> insert into t2 values ('006','SanFran','Itisasanfranlifestyle');
> insert into t2 values ('007','BerkAuthor','Cool.Berkley.the.book');
> insert into t3 values('000','NewYorkPublicLibra',1);
> insert into t3 values('001','NewYorkPublicLibra',2);
> insert into t3 values('002','NewYorkPublicLibra',3);
> insert into t3 values('003','NewYorkPublicLibra',4);
> insert into t3 values('004','NewYorkPublicLibra',5);
> insert into t3 values('005','NewYorkPublicLibra',6);
> insert into t3 values('006','SanFransiscoPublic',5);
> insert into t3 values('007','BerkeleyPublic1',3);
> insert into t3 values('007','BerkeleyPublic2',3);
> insert into t3 values('001','NYC Lib',8);
> insert into t1 values ('NewYorkPublicLibra','NewYork');
> insert into t1 values ('SanFransiscoPublic','SanFran');
> insert into t1 values ('BerkeleyPublic1','Berkeley');
> insert into t1 values ('BerkeleyPublic2','Berkeley');
> insert into t1 values ('NYCLib','NewYork');
> select * from (select city,libname1,count(libname1) as a from t3 join t1 on libname1=libname3 join t2 on isbn3=isbn2 group by city,libname1) sub ;
+----------------------------------+
| city | libname1 | a |
+----------------------------------+
| NewYork | NewYorkPublicLibra | 6 |
| SanFran | SanFransiscoPublic | 1 |
| Berkeley | BerkeleyPublic1 | 1 |
| Berkeley | BerkeleyPublic2 | 1 |
+----------------------------------+
4 rows in set (0.00 sec)
```````` sql
create table t1 (libname1 varchar(21) not null primary key, city varchar(20));
create table t2 (isbn2 varchar(21) not null primary key, author varchar(20), title varchar(60));
create table t3 (isbn3 varchar(21) not null, libname3 varchar(21) not null, quantity int);
insert into t2 values ('001','Daffy','Aducklife');
insert into t2 values ('002','Bugs','Arabbitlife');
insert into t2 values ('003','Cowboy','Lifeontherange');
insert into t2 values ('000','Anonymous','Wannabuythisbook?');
insert into t2 values ('004','BestSeller','OneHeckuvabook');
insert into t2 values ('005','EveryoneBuys','Thisverybook');
insert into t2 values ('006','SanFran','Itisasanfranlifestyle');
insert into t2 values ('007','BerkAuthor','Cool.Berkley.the.book');
insert into t3 values('000','NewYorkPublicLibra',1);
insert into t3 values('001','NewYorkPublicLibra',2);
insert into t3 values('002','NewYorkPublicLibra',3);
insert into t3 values('003','NewYorkPublicLibra',4);
insert into t3 values('004','NewYorkPublicLibra',5);insert into t3 values('005','NewYorkPublicLibra',6);
insert into t3 values('006','SanFransiscoPublic',5);
insert into t3 values('007','BerkeleyPublic1',3);
insert into t3 values('007','BerkeleyPublic2',3);
insert into t3 values('001','NYC Lib',8);
insert into t1 values ('NewYorkPublicLibra','NewYork');
insert into t1 values ('SanFransiscoPublic','SanFran');
insert into t1 values ('BerkeleyPublic1','Berkeley');
insert into t1 values ('BerkeleyPublic2','Berkeley');
insert into t1 values ('NYCLib','NewYork');

mysql> select * from (select city,libname1,count(libname1) as a from t3 join t1 on libname1=libname3 join t2 on isbn3=isbn2 group by city,libname1) sub ;
+----------+--------------------+------+
| city     | libname1           | a    |
+----------+--------------------+------+
| NewYork  | NewYorkPublicLibra |    6 |
| SanFran  | SanFransiscoPublic |    1 |
| Berkeley | BerkeleyPublic1    |    1 |
| Berkeley | BerkeleyPublic2    |    1 |
+----------+--------------------+------+
4 rows in set (0.00 sec)
```