194 lines
7.7 KiB
Markdown
194 lines
7.7 KiB
Markdown
- #[[CT230 - Database Systems I]]
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- **Previous Topic:** [[Introduction to SQL & DDL]]
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- **Next Topic:** [[SQL DML: SELECT]]
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- **Relevant Slides:** 
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-
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- # Notes on Syntax
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- While SQL is case insensitive, Linux ^^is not.^^
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- web1.cs.nuigalway.ie is a ^^linux server^^, so we need to be careful with table names in particular, as `EMPLOYEE` is ^^not the same thing^^ as `employee`.
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- Attribute names are separated by commas.
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- Strings are enclosed in quotes.
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- Numbers are not enclosed in quotes.
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-
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- Usually, SQL keywords are capitalised while table & keyword names are mostly kept in lowercase unless using camelCase instead of snake_case.
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- Code should be organised horizontally & vertically, and not all written on one line.
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- Code blocks are separated by a semicolon.
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- Comments can be made using `#`, `--`, & `/* */`.
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-
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- # **CRUD** Operations in DML
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- What are **CRUD** Operations? #card
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card-last-interval:: 31.36
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card-repeats:: 4
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card-ease-factor:: 2.8
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card-next-schedule:: 2022-12-16T04:09:37.257Z
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card-last-reviewed:: 2022-11-14T20:09:37.258Z
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card-last-score:: 5
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- **CRUD** operations are the 4 basic functions that we may wish to perform on *persistent* data.
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- **Create:** insert a new tuple. `INSERT`
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- **Read:** retrieve some data. `SELECT`
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- **Update:** modify some data. `Update`
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- **Delete:** delete some data or a tuple. `DELETE`
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- ## Read: `SELECT`
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- The basic syntax for an SQL select query to *read* data consists of 6 clauses. #card
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collapsed:: true
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card-last-interval:: 19.01
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card-repeats:: 4
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card-ease-factor:: 2.18
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card-next-schedule:: 2022-12-03T20:06:02.473Z
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card-last-reviewed:: 2022-11-14T20:06:02.473Z
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card-last-score:: 3
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- ```sql
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SELECT [DISTINCT] <attribute list>
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FROM <table list>
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WHERE <condition>
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GROUP BY <group attributes>
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HAVING <group condition>
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ORDER BY <attribute list>
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```
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- The order of these clauses ^^cannot be changed.^^
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- `SELECT` & `FROM` are ^^always required^^, other clauses are optional.
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- ### `SELECT <attribute list> FROM <table list> WHERE <condition>` #card
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collapsed:: true
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card-last-interval:: 108
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card-repeats:: 5
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card-ease-factor:: 3
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card-next-schedule:: 2023-03-06T18:36:40.152Z
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card-last-reviewed:: 2022-11-18T18:36:40.153Z
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card-last-score:: 5
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- `<attribute list>` is a list of **attribute** (column) names (separated by commas) whose values will be retrieved by the query.
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- `<table list>` is a list of table names (separated by commas) containing the attributes.
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- `<condition>` is a **Boolean** expression that identifies the tuples to be retrieved by the query.
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- For each **tuple** (row) in the table(s) which are part of the query:
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- the tuple is checked to see if the condition is **true** for this tuple.
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- If **true**, the tuple **is** part of the output.
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- If **false**, the tuple is **not** part of the output.
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- The comparison operators are:
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- `=` `<=` `<` `>` `>=` `!=`
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- Conditions can be compounded by use of Boolean `AND`, `OR`, and can be negated with `NOT`.
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- ^^Note:^^ In some versions of SQL, (e.g., in MS), the `!=` operator is written as `<>`.
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- ### Calculated or Derived Fields #card
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collapsed:: true
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card-last-interval:: 28.3
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card-repeats:: 4
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card-ease-factor:: 2.66
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card-next-schedule:: 2022-12-15T16:48:02.726Z
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card-last-reviewed:: 2022-11-17T09:48:02.727Z
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card-last-score:: 5
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- We can specify an SQL expression in the `SELECT` clause which can involve **numerical operations** on **numeric** fields and **counting operations** on **non-numeric** fields.
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- e.g.,
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- ```sql
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SELECT ssn, salary/12 FROM employee;
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```
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- ### Tidying Up the Output #card
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card-last-interval:: 19.01
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card-repeats:: 4
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card-ease-factor:: 2.18
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card-next-schedule:: 2022-12-03T20:05:39.227Z
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card-last-reviewed:: 2022-11-14T20:05:39.227Z
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card-last-score:: 3
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- We can use keywords `CAST`, `AS`, & `DECIMAL(x, y)` to specify the total number of digits `x` and the number of digits after the decimal point `y` when we're working with **real numbers**.
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- ```sql
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SELECT ssn, CAST(salary/12.0 AS DECIMAL(8,2))
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FROM employee;
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```
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- We can also use the keyword `AS` to rename output.
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- ```sql
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SELECT ssn, CAST(salary/12.0 AS DECIMAL(8,2))
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AS monthlySalary
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FROM employee;
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```
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-
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- ### Keyword `DISTINCT` #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:: 2022-12-21T00:50:04.359Z
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card-last-reviewed:: 2022-11-17T09:50:04.360Z
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card-last-score:: 5
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- The keyword `DISTINCT` automatically removes duplicates from the returned result set.
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- We should be careful of using with large result sets as it can be an expensive operation to perform (not a problem for our small examples).
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- ```sql
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SELECT DISTINCT salary
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FROM employee;
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```
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- ### Selecting All Attribute Values for Selected Tuples #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:: 2022-11-25T20:36:44.458Z
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card-last-reviewed:: 2022-11-14T16:36:44.458Z
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card-last-score:: 5
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- To retrieve all attribute values of selected tuples, you do not have to explicitly list all the attribute names - instead, you can use `SELECT *`.
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- ### New Operators
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collapsed:: true
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- `BETWEEN` #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:: 2022-12-18T11:07:14.075Z
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card-last-reviewed:: 2022-11-14T20:07:14.075Z
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card-last-score:: 5
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- The `BETWEEN` operator selects values within a given range. The values can be numbers, texts, or dates.
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- ```sql
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SELECT <column name(s)>
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FROM <table name>
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WHERE <column name(s)> BETWEEN <value 1> AND <value 2>;
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```
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- `IN` #card
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card-last-interval:: -1
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card-repeats:: 1
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card-ease-factor:: 2.56
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card-next-schedule:: 2022-11-18T00:00:00.000Z
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card-last-reviewed:: 2022-11-17T09:48:15.246Z
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card-last-score:: 1
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- The `IN` operator tests if a data value matches one of a list of values.
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- It is a shorthand for multiple `OR` conditions.
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- ```sql
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SELECT <column name(s)>
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FROM <table_name>
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WHERE <column_name> IN (<value1>, <value2>, ...);
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-- OR --
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SELECT <column_name(s)>
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FROM <table_name>
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WHERE <column_name> IN (SELECT <statement>);
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```
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- `LIKE` #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:: 2022-12-21T00:50:18.687Z
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card-last-reviewed:: 2022-11-17T09:50:18.687Z
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card-last-score:: 5
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- Allows string comparison, when equality is too strict.
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- There are 2 wildcards, often used in conjunction with the `LIKE` operator:
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- The percent sign `%` represents zero, one, or multiple characters.
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- The underscore sign `_` represents one, single character.
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- ```sql
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SELECT <column_name(s)>
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FROM <table_name>
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WHERE <column_name> LIKE <pattern>;
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```
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- `IS NULL` #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:: 2022-12-18T07:38:30.380Z
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card-last-reviewed:: 2022-11-14T16:38:30.380Z
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card-last-score:: 5
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- Allows an explicit search for `NULL`.
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-
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-
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- ### Set Operators
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- `UNION`
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- `INTERSECTION`
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- `MINUS / DIFFERENCE`
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-
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- ```SQL
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SELECT full_name, ssn FROM (
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SELECT ssn, CONCAT(fname, " ", minit, " ", lname)
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AS full_name from employee) AS dtable
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WHERE full_name IN ("John B Smith", "James E Borg", "Joyce A English");
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``` |