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