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Rename year directories to allow natural ordering

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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
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- Used to rename any output in `SELECT`.
- Can also be used to **alias** (rename) tables in `FROM`.
- `CONCAT` #card
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- 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
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- `CAST(<expression> AS <datatype>(<length>))`
- Used to cast to another datatype.
- `ORDER BY` #card
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- `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
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- 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
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- 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
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- 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
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- 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
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- What methods can you use for accessing data across multiple tables? #card
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- There are 3 potential approaches:
- Joins.
- Subqueries.
- Union queries.
- ## Subqueries #card
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- 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
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- 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).
-
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