2.sql select

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Questions and Answers

What is the purpose of the SQL SELECT statement?

The SQL SELECT statement is used to retrieve data from one or more tables.

Explain the use of the WHERE clause in an SQL query.

The WHERE clause is used to specify a condition that determines which records should be selected.

In an SQL INSERT statement, what happens if you do not specify column names?

If column names are not specified, the values must match the order of all columns in the table.

What happens when you execute an SQL UPDATE statement without a WHERE clause?

Executing an UPDATE statement without a WHERE clause will update all records in the table. Signup and view all the answers

How does the ORDER BY clause affect the result set of an SQL query?

The ORDER BY clause is used to sort the result set by one or more specified columns. Signup and view all the answers

What is the function of the INSERT command in SQL?

The INSERT command is used to add new records or entries into a specified table. Signup and view all the answers

Define the GROUP BY clause in SQL.

The GROUP BY clause is used to group rows that have the same values in specified columns. Signup and view all the answers

What does the HAVING clause do in an SQL query?

The HAVING clause is used to filter groups based on conditions after aggregation. Signup and view all the answers

Explain what happens when the condition in a DELETE statement is omitted.

If the condition is omitted in a DELETE statement, all records in the table will be deleted. Signup and view all the answers

Describe how the WHERE clause functions in an SQL statement.

The WHERE clause specifies conditions that must be met for records to be selected or affected. Signup and view all the answers

What SQL syntax is used to select records within a range of values?

The syntax used is 'column_name BETWEEN value1 AND value2'. Signup and view all the answers

How does the IN operator enhance SQL queries?

The IN operator allows specification of multiple discrete values for comparison in a WHERE clause. Signup and view all the answers

What is the significance of using the LIKE operator in SQL?

The LIKE operator is used for pattern matching, allowing selection of records based on partial values. Signup and view all the answers

Illustrate how NULL values can be handled in SQL queries.

NULL values can be identified using 'column_name IS NULL' to find records without values in that column. Signup and view all the answers

Can multiple rows be updated in an SQL UPDATE statement? Provide an example.

Yes, multiple rows can be updated if they meet the specified condition in the UPDATE statement. Signup and view all the answers

Study Notes

Database Usage, SQL

Database usage involves using data and making corrections.
SQL is used in database management.

DML - Data Manipulation Statements

SELECT: Retrieves data from one or multiple tables.
INSERT: Adds new records to one or more tables.
UPDATE: Modifies existing records in one or multiple tables.
DELETE: Removes existing records from one or more tables.
FROM: Specifies which tables to retrieve data from.
WHERE: Filters the data based on a condition.
GROUP BY: Groups results based on one or more columns.
ORDER BY: Sorts the results based on one or more columns.
HAVING: Filters grouped results based on a condition.

SQL SELECT Statement

Used to retrieve data from a table or tables.
The result is a "result-set".
Syntax:
- SELECT column1, column2, ... FROM table_name;
- SELECT * FROM table_name; (* selects all columns)

Example SQL SELECT Statement

Example query using DARBUOTOJAI table.
Shows columns ASMENS_KODAS, VARDAS, PAVARDE, DIRBA_NUO, ATLYGINIMAS, PARDUOTUVES_ID.
Possible selection based on a specific filter.

SQL WHERE Clause

Used to filter rows based on specified criteria.
Used in SQL queries.
Syntax:
- WHERE condition
Example: Retrieve data where 'name' is equal to 'Antanas'

Example SQL WHERE Statement

Examples using DARBUOTOJAI table.
SELECT VARDAS, ATLYGINIMAS FROM DARBUOTOJAI WHERE VARDAS='Antanas'

SQL INSERT Statement

Used to add new rows to a table.
Syntax:
- INSERT INTO table_name (column1, column2, ...) VALUES (value1, value2, ...);
Can insert multiple rows at once.

SQL UPDATE Statement

Used to modify existing rows.
Syntax: UPDATE table_name SET column1 = value1, column2 = value2 WHERE condition;
Optionally uses WHERE clause for conditional updates (important! changes all matching rows if it's omitted).

SQL DELETE Statement

Used to remove rows from a table.
Syntax:
- DELETE FROM table_name WHERE condition;
TRUNCATE removes all rows.
DROP removes the entire table structure.

SQL WHERE Clause (Operators)

Used to filter rows based on various operators.
Examples:
- =, >, <, >=, <=, <>, !=, IN, NOT IN, BETWEEN, LIKE.

SQL WHERE Clause (BETWEEN...AND)

Filters records within a range.
Use with numerical or date columns.

SQL WHERE Clause (IN)

Used for multiple-value comparisons.

SQL WHERE Clause (LIKE)

Tests values for pattern matches.
Uses wildcards (%, _) for pattern matching.

SQL WHERE Clause (NULL)

Checks if a column value is NULL.
The IS NULL operator is used.

SQL WHERE Clause (NOT, AND, OR)

Combines multiple conditions using logical operators.

SQL WHERE Clause (Precedence)

The order to apply logical conditions.
Precedence of operators in SQL WHERE clause.

SQL ORDER BY Clause

Sorts query results.
Orders results ascending or descending (ASC/DESC).

SQL Other Functions

DISTINCT: Selects unique values.
LOWER/UPPER: Converts to lowercase/uppercase.
CONCAT: Joins strings together.
COUNT, SUM, AVG, MIN, MAX are aggregate functions for calculations on a set of rows.

SQL GROUP BY

Groups rows with same values in specific columns.
Can be used with aggregate functions.

SQL HAVING Clause

Filters grouped results.

SQL Syntax (summary)

A concise summary of the fundamental components of SQL syntax.

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