Basic select statement

SELECT …

SELECT 1;  --  return 1
SELECT 1+1;--  return 2
SELECT 1*3;--  return 3

SELECT … FROM

• DUAL It's a pseudo table .
• SELECT Field 1, Field 2,... Field n FROM Table name .
• SELECT * FROM Table name . among * representative All fields in the table （ Or all columns ）.

SELECT 1
FROM DUAL;
--  among ,DUAL It's a pseudo table .

DESCRIBE DUAL;
--  Report errors ：You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'DUAL' at line 1

SELECT *
FROM employees;
-- *, representative   All fields in the table （ Or all columns ）

SELECT DISTINCT … （ Remove duplicate lines ）

• SELECT DISTINCT, Remove duplicate lines .
• SELECT DISTINCT department_id, Remove department_id Repeated lines .
• SELECT DISTINCT department_id,salary, Remove department_id、salary Lines repeated at the same time .

SELECT department_id
FROM employees; 
--  return 107 Bar record 

#  Remove duplicate lines 
SELECT DISTINCT department_id
FROM employees;
--  return 12 Bar record 

#  The negative case 
SELECT DISTINCT department_id
FROM employees; 
--  return 12 rows 

SELECT salary,department_id
FROM employees;
--  return 106 rows 

SELECT salary,DISTINCT department_id
FROM employees;
--  Report errors 
/* You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'DISTINCT department_id FROM employees' at line 1 */

#  The negative case 
SELECT department_id,salary
FROM employees;
--  return 107 rows 

SELECT DISTINCT department_id,salary
FROM employees;
--  return 74 rows 

SELECT department_id,salary
FROM employees
WHERE department_id = 90 
AND salary = 17000;
/*  return 2 rows , as follows ： department_id salary 90 17000.00 90 17000.00 */

SELECT DISTINCT department_id,salary
FROM employees
WHERE department_id = 90
AND salary = 17000;
/*  return 1 rows , as follows , department_id salary 90 17000.00  namely  SELECT DISTINCT department_id,salary  Removed  department_id、salary Duplicate records at the same time . */

Alias of column

Alias the column , There are the following 3 Ways of planting ,

1. Name Alias , such as SELECT employee_id emp_id FROM employees;
2. Name AS Alias , such as SELECT employee_id AS emp_id FROM employees;
3. Name “ Alias ”, such as SELECT employee_id "emp_id" FROM employees;

SELECT employee_id emp_id 
FROM employees;

SELECT department_name AS dep_name
FROM departments;

SELECT salary*12 "annual salary"
FROM employees;

Null values participate in the operation

Null value , namely NULL. Null values participate in the operation , The result is still null .

SELECT employee_id,salary " Monthly wages ",commission_pct,12*salary*(1+commission_pct) " Annual income "
FROM employees;
--  When commission_pct by NULL when ,12*salary*(1+commission_pct) The result of the operation is still NULL.

Flow control function

• IF(condition,value1,value2), If condition by true, Then return to value1, Otherwise return to value2.
• IFNULL(value1,value2), If value1 Not for NULL, Then return to value1, Otherwise return to value2.

SELECT employee_id,salary " Monthly wages ",commission_pct,12*salary*(1+IF(commission_pct IS NOT NULL,commission_pct,0)) " Annual income "
FROM employees;

SELECT employee_id,salary " Monthly wages ",commission_pct,12*salary*(1+IFNULL(commission_pct,0)) " Annual income "
FROM employees;

LIMIT grammar

LIMIT grammar , There are two ,

• LIMIT Offset , Number of entries , such as ,SELECT ... FROM ... LIMIT 44,2, The offset 44,2 Bar record .
• LIMIT Number of entries OFFSET Offset . such as ,SELECT ... FROM ... LIMIT 2 OFFSET 44, The offset 44,2 Bar record .

SELECT employee_id,salary " Monthly wages ",commission_pct,12*salary*(1+IF(commission_pct IS NOT NULL,commission_pct,0)) " Annual income "
FROM employees 
LIMIT 2 OFFSET 44;

SELECT employee_id,salary " Monthly wages ",commission_pct,salary*(1+IFNULL(commission_pct,0)) " Annual income "
FROM employees 
LIMIT 2 OFFSET 44;

SELECT employee_id,salary " Monthly wages ",commission_pct,salary*(1+IF(commission_pct IS NOT NULL,commission_pct,0)) " Annual income "
FROM employees 
LIMIT 44,2;

/*  return 2 rows  employee_id  Monthly wages  commission_pct  Annual income  144 2500.00 （Null） 30000.00 145 14000.00 0.40 235200.00 */

mark of emphasis （``）

to database 、 surface 、 Field When naming , The database name must be guaranteed 、 Table name 、 Field name , Do not reserve words 、 Database systems or common methods conflict . In case of conflict , stay SQL Use emphasis marks in statements （``） Lead up .

#  Error model 
SELECT *
FROM order;
/*  An error is as follows , Because the table name order  And  MySQL keyword (ORDER BY) The conflict  You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'order' */

#  Correct demonstration 
#  Use emphasis ``
SELECT *
FROM `order`;

Query constant

SELECT ' Happy Lianmeng ',employee_id,department_id,last_name
FROM employees
LIMIT 3;

/*  return 3 rows , as follows ：  Happy Lianmeng  employee_id department_id last_name  Happy Lianmeng  100 90 King  Happy Lianmeng  101 90 Kochhar  Happy Lianmeng  102 90 De Haan */

Display table structure

Display table structure , The grammar is as follows ：

• DESC Table name . such as ,DESC employees;
• DESCRIBE Table name . such as ,DESCRIBE employees;