SQL · Theory

Select Database

Theory

9/439

Select Database

USE college; selects which database later statements talk to. It does not print the table. Think: open this cupboard, then look at registers inside.

In Workbench the schema you highlight is the same idea. In an app, the connection already names the database, so you often skip USE.

Check — SELECT DATABASE(); shows the one in use. NULL means you did not select any.

Trap — running SELECT * FROM students without USE, and MySQL says ‘No database selected’.

On the example next to this theory — Select Database — sELECT projects columns; WHERE filters; ORDER BY sorts.

Diagram

  FROM students
        │
        ▼
  WHERE city = 'Pune'
        │
        ▼
  SELECT name, marks

Exam tip

USE college; then SELECT DATABASE();

Select Database — sample query

CREATE TABLE employees (id INTEGER PRIMARY KEY, name TEXT, dept TEXT, salary INTEGER);
INSERT INTO employees VALUES
  (1, 'Asha', 'Eng', 70000),
  (2, 'Ravi', 'Eng', 65000),
  (3, 'Neha', 'HR', 50000);
SELECT name, salary FROM employees WHERE dept = 'Eng' ORDER BY salary DESC;

Select Database — sELECT projects columns; WHERE filters; ORDER BY sorts.

Short notes

DefUSE dbname; picks the database for the session.
RuleSELECT DATABASE(); to check.
TrapQuery without USE → no database selected.

SQL · Theory

Select Database

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Theory

9/439

Select Database

USE college; selects which database later statements talk to. It does not print the table. Think: open this cupboard, then look at registers inside.

In Workbench the schema you highlight is the same idea. In an app, the connection already names the database, so you often skip USE.

Check — SELECT DATABASE(); shows the one in use. NULL means you did not select any.

Trap — running SELECT * FROM students without USE, and MySQL says ‘No database selected’.

On the example next to this theory — Select Database — sELECT projects columns; WHERE filters; ORDER BY sorts.

Diagram

  FROM students
        │
        ▼
  WHERE city = 'Pune'
        │
        ▼
  SELECT name, marks

Exam tip

USE college; then SELECT DATABASE();

Select Database — sample query

CREATE TABLE employees (id INTEGER PRIMARY KEY, name TEXT, dept TEXT, salary INTEGER);
INSERT INTO employees VALUES
  (1, 'Asha', 'Eng', 70000),
  (2, 'Ravi', 'Eng', 65000),
  (3, 'Neha', 'HR', 50000);
SELECT name, salary FROM employees WHERE dept = 'Eng' ORDER BY salary DESC;

Select Database — sELECT projects columns; WHERE filters; ORDER BY sorts.

Short notes

DefUSE dbname; picks the database for the session.
RuleSELECT DATABASE(); to check.
TrapQuery without USE → no database selected.

Select Database

Select Database — sample query

Short notes

Questions

Select Database

Select Database — sample query

Short notes

Questions