Sql Subquery

What is an Sql Subquery?

It's a nested Sql Select inside the WHERE clause of another Sql Select statement. A subquery itself is a full Sql statement, and it needs to be enclosed by open and close parentheses, and it is also referred to as inner subquery, while the next level up of a subquery is the outer level.

As previously mentioned in the Sql In that to specify multiple values is Sql Where, besides using a list of values, you can also use sql subquery.

The syntax:

SELECT  column_list
WHERE column_name
} outer subquery
     [NOT] IN
          SELECT  column_list
          FROM table_name
          WHERE conditions
} inner subquery



Sql subquery is a simple concept, although the syntax seems complicated. Let's see the following example.

You have a big table, Tbl_L. Also you have another similar table but smaller,Tbl_R.

Table Tbl_L
Table Tbl_R

You want to find all entry in tbl_L that are also in tbl_R. This is how you do it.

Sql Statement:

--use '*' to select all

--'ID' data type must be compatible
--with ID in the inner query
          SELECT  ID
          FROM Tbl_R
--select only single column




You could also formulate this sql subquery as a join query(see Sql Join).

Sql Statement:

SELECT tbl_l.*,
FROM   tbl_l,
WHERE  tbl_l.id = tbl_r.id 



Use Sql Join if you need to include columns from both tables.

Quick link. From here, you can learn the following:

Data Creation

Sql Create Database
Use this to create a database.

Sql Create Table
This Sql Create Table will be used very often throughout the tutorial. It is used to create a table, in our case, mainly to create sample tables.

Sql Alter Table
Used less often, but it is very handy to know this command. With this you can basically edit a table structure.

Sql Drop Table
Used to delete a table, not just the content. This command is also used very often during the tutorial. So, you need to master this too.

Data Manipulation

Sql Insert
To add rows to a table, you use Sql Insert command. There are two methods, one is with the VALUES keyword and second is with a SELECT statement.

Sql Update
To change existing rows, you use Sql Update command. You can use it to change values in a single row, groups of rows or all rows. You can also update a table based on values in other tables.

Sql Delete
To remove rows, you use Sql Delete command. You can delete one rows, group of rows or all rows. You can also delete rows based on values in other tables.

Data Retrieval

Sql Select
The core of Sql, selecting data from table.

Sql Distinct
Eliminating duplicate values in the output.

Sql Where
Conditionally selecting rows from table.

Sql Like
Pattern searching for conditions in Where or Having clause

Sql Between
Added to conditions to show results between two values.

Sql In
It enables us to specify multiple values, either as a list or a subquery.

Sql And Or
Connecting two or more conditions in the Where or Having clause.

Sql Count
Counting number of rows.

Sql Sum
Getting the sum of a field

Sql Max
This function will return the highest value.

Sql Min
This function will return the lowest value.

Sql Avg
This function will return the average value.

Sql Order By
Sorting the output

Sql Group By
Grouping rows into sets.

Sql Having
To filter the result of aggregation

Advanced Sql Tutorial

Sql Inner Join
Use Sql Join to retrieve data from more than one table in just one Select statement.

Sql Outer Join
An Sql outer join shows the join, and all the rows that do not qualify.

Sql Self Join
A join that compares values within a single table.

Sql Union
Merging two or more queries into one result. Rows are appended from one queries to the other.

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