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.
| [NOT] IN
| SELECT column_list
Sql subquery is a simple concept, although the syntax seems complicated. Let's see the following example.
You want to find all entry in tbl_L that are also in tbl_R. This is how you do it.
|--use '*' to select all
--'ID' data type must be compatible
--with ID in the inner query
| SELECT ID
|--select only single column
You could also formulate this sql subquery as a join query(see Sql Join).
Use Sql Join if you need to include columns from both tables.
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.
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.
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.
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.
The core of Sql, selecting data from table.
Eliminating duplicate values in the output.
Conditionally selecting rows from table.
Pattern searching for conditions in Where or Having clause
Added to conditions to show results between two values.
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.
Counting number of rows.
Getting the sum of a field
This function will return the highest value.
This function will return the lowest value.
This function will return the average value.
Sql Order By
Sorting the output
Sql Group By
Grouping rows into sets.
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.
Merging two or more queries into one result. Rows are appended from one queries to the other.
Return from Sql Subquery to Advanced Sql Tutorial
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