title:: SQL SELECT: Working with Strings & Subqueries

- #[[CT230 - Database Systems I]]
- **Previous Topic:** [[SQL DML: SELECT]]
- **Next Topic:** [[Aggregate Clauses, Group By, & Having Clauses]]
- **Relevant Slides:** ![Topic 5 DML SELECT strings and subqueries.pdf](../assets/Topic_5_DML_SELECT_strings_and_subqueries_1664273683998_0.pdf)
-
- # Keywords to Modify Output
	- `AS` #card
	  card-last-interval:: -1
	  card-repeats:: 1
	  card-ease-factor:: 2.7
	  card-next-schedule:: 2022-11-15T00:00:00.000Z
	  card-last-reviewed:: 2022-11-14T16:35:37.700Z
	  card-last-score:: 1
		- Used to rename any output in `SELECT`.
		- Can also be used to **alias** (rename) tables in `FROM`.
	- `CONCAT` #card
	  card-last-interval:: 11.2
	  card-repeats:: 3
	  card-ease-factor:: 2.8
	  card-next-schedule:: 2022-11-25T20:35:14.265Z
	  card-last-reviewed:: 2022-11-14T16:35:14.265Z
	  card-last-score:: 5
		- Used to **concatenate** strings.
		- Similar usage to other programming languages.
		- ```sql
		  SELECT
		  	CONCAT(fname, ' ', minit, ' ', lname) AS Name
		  FROM
		  	employee
		  WHERE
		  	salary BETWEEN 50000 AND 80000
		  ORDER BY
		  	lname;
		  ```
	- `CAST` #card
	  card-last-interval:: 33.64
	  card-repeats:: 4
	  card-ease-factor:: 2.9
	  card-next-schedule:: 2022-12-18T11:21:08.757Z
	  card-last-reviewed:: 2022-11-14T20:21:08.757Z
	  card-last-score:: 5
		- `CAST(<expression> AS <datatype>(<length>))`
		- Used to cast to another datatype.
	- `ORDER BY` #card
	  card-last-interval:: 11.2
	  card-repeats:: 3
	  card-ease-factor:: 2.8
	  card-next-schedule:: 2022-11-25T20:37:08.438Z
	  card-last-reviewed:: 2022-11-14T16:37:08.438Z
	  card-last-score:: 5
		- `ORDER BY <attribute list> <order (ASC or DESC)> `.
		- Allows the results of a query to be ordered by values of one or more attributes.
		- Either **ascending** `ASC` or **descending** `DESC` - ascending by default.
		- Must be the *last* clause of a `SELECT` statement.
- # `TOP` & `LIMIT` #card
  card-last-interval:: 10.97
  card-repeats:: 3
  card-ease-factor:: 2.56
  card-next-schedule:: 2022-11-25T15:38:08.480Z
  card-last-reviewed:: 2022-11-14T16:38:08.481Z
  card-last-score:: 5
	- The `SELECT TOP <N>` clause is used to specify the number of tuples (N) to return, but it is not supported by MySQL.
	- Instead, MySQL supports a `LIMIT <N>` clause which has the same functionality.
	- The `LIMIT` clause is listed at the end of the query,
	- Example: List the employees with the top 3 salaries.
		- ```sql
		  SELECT
		  	ssn, CONCAT(fname, ' ', lname) as Name, salary
		  FROM
		  	employee
		  ORDER BY
		  	salary DESC
		  LIMIT 4;
		  ```
- # Strings
	- ## Note: 'Single' & "Double" Quotes #card
	  card-last-interval:: 23.43
	  card-repeats:: 4
	  card-ease-factor:: 2.42
	  card-next-schedule:: 2022-12-08T06:20:40.093Z
	  card-last-reviewed:: 2022-11-14T20:20:40.094Z
	  card-last-score:: 5
		- MySQL usually allows single & double quotes to be used interchangably.
		- Generally, single quotes should be used for strings (`varchar()`, `text`, etc.).
		- ### How to Deal with Apostrophes in Strings #card
		  card-last-interval:: 33.64
		  card-repeats:: 4
		  card-ease-factor:: 2.9
		  card-next-schedule:: 2022-12-18T11:21:10.692Z
		  card-last-reviewed:: 2022-11-14T20:21:10.692Z
		  card-last-score:: 5
			- You have to be careful with apostrophes in strings, as an opening quote could be accidentally closed by an apostrophe.
			- To overcome this, if there is an apostrophe in a string, it should be replaced by **two** apostrophes side-by-side `''` (this is a general rule for all special characters - have two of the character) or escape it with `\`.
			- Example: Select the salary for the employee with surname O'Grady.
				- ```SQL
				  SELECT salary
				  FROM employee
				  WHERE lname = 'O''Grady';
				  ```
			- We must also take care when inserting string data using `INSERT INTO`.
	- ## Working with Strings & Pattern Matching #card
	  card-last-interval:: 4.14
	  card-repeats:: 2
	  card-ease-factor:: 2.56
	  card-next-schedule:: 2022-11-22T21:33:52.858Z
	  card-last-reviewed:: 2022-11-18T18:33:52.859Z
	  card-last-score:: 5
		- SQL is case insensitive (apart from table names if on a Linux server).
		- Case insensitivity generally applies to string searching.
		- However, *often* when working with strings we do not look for an exact match (i.e., an exact match using `=`).
		- To support partial matching, we often use **pattern-matching characters** and `LIKE` with wildcard characters `%` and `_`.
			- `%` represents 0 or more characters.
			- `_` Represents a single character.
		- ### Regexp
			- We can use regex for more complicated string matching.
			- ```sql
			  SELECT <attribute>
			  FROM <relation>
			  WHERE <attribute> REGEXP <regexp>
			  ```
			- `^` Matches position at the **beginning** of the searched string.
			- `$` Matches position at the **end** of the searched string.
			- `[]` Matches any character inside the square brackets.
			- `[^]` Matches any character **not** inside the square brackets.
			- `*` Matches preceeding character 0 or more times.
			- `+` Matches preceeding character 1 or more times.
			- `|` OR.
			- `{n}` Matches preceeding character n number of times.
- # Accessing Data Across Multiple Tables
  card-last-score:: 5
  card-repeats:: 2
  card-next-schedule:: 2022-10-11T03:36:21.816Z
  card-last-interval:: 3.71
  card-ease-factor:: 2.46
  card-last-reviewed:: 2022-10-07T10:36:21.817Z
	- What methods can you use for accessing data across multiple tables? #card
	  card-last-interval:: 4
	  card-repeats:: 2
	  card-ease-factor:: 2.7
	  card-next-schedule:: 2022-11-18T20:10:10.085Z
	  card-last-reviewed:: 2022-11-14T20:10:10.085Z
	  card-last-score:: 5
		- There are 3 potential approaches:
			- Joins.
			- Subqueries.
			- Union queries.
	- ## Subqueries #card
	  card-last-interval:: 11.2
	  card-repeats:: 3
	  card-ease-factor:: 2.8
	  card-next-schedule:: 2022-11-25T20:36:08.786Z
	  card-last-reviewed:: 2022-11-14T16:36:08.786Z
	  card-last-score:: 5
		- A **subquery** (also called a **nested** query)is a query with another query.
		- The subquery *usually* returns data that will be used in the main query.
		- Data returned from the subquery may be a **set of values** or a **single value**.
		- Subqueries can be used with the `SELECT`, `INSERT`, `UPDATE`, and `DELETE` statements.
		- ![image.png](../assets/image_1664275471660_0.png)
		- The `SELECT` statement that contains a subquery is called an **outer query**.
		- ### Connecting Inner & Outer Queries #card
		  card-last-interval:: -1
		  card-repeats:: 1
		  card-ease-factor:: 2.08
		  card-next-schedule:: 2022-11-15T00:00:00.000Z
		  card-last-reviewed:: 2022-11-14T20:19:14.578Z
		  card-last-score:: 1
			- If a subquery returns only **one value** then we can use operators such as `=`, `!=`. `>`, `>=`, `<`, `<=`.
			- If a subquery *could* return **more than one value** (i.e., a list of values), then we need connectors such as `IN`, `ANY`, `ALL` to check through the values of the subquery.
				- The keyword `NOT` can also be used where appropriate.
				- These connectors are generally used with basic algebraic operators: `=`, `!=`. `>`, `>=`, `<`, `<=`.
				- `ALL` - the condition is true if the comparison is true for **every** (all) values returned by the subquery.
				- `ANY` - the condition is true if the comparison is true for **at least one** (any) value returned by the subquery.
				- `IN` - the condition is true if the comparison is true for **at least one** value returned by the subquery, i.e., a value IN the subquery.
					- `IN` checks for equality - it can be used for a list of values or a single value - it does not require any additional algebraic operator.
			- In addition, we can have a more general condition using:
				- `EXISTS` -  True if there exists at least one value in the result from a subquery.
				- `NOT EXISTS` - True if there is nothing in the result from a subquery (i.e., it is empty).
				-
-