Please see the manual for your database manager and have fun trying different options yourself. I hope this at least gives you enough to get started. *As with all of these SQL things there is MUCH MORE to them than what’s in this introductory guide.
Where (studentID between 1 and 5 or studentID = 8) Here’s an updated query, where any record that has an SAT score that’s in this list (1000, 1400) will not be presented: select studentID, FullName, sat_score, recordUpdated
| studentID | FullName | sat_score | rcd_updated |
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Here is the current full student list to compare to the WHERE clause result set: select studentID, FullName, sat_score, rcd_updated from student +-+-+-+-+ In this case all five of these will be used is a somewhat ridiculous WHERE clause. The WHERE clause is used to limit the number of rows returned. SQL Server / Oracle / MS Access: ALTER TABLE PersonsĭROP CHECK CHK_PersonAge SQL Where Clause WHERE Clause (and/or, IN, BETWEEN, and LIKE) To drop a CHECK constraint, use the following SQL: MySQL / SQL Server / Oracle / MS Access: ALTER TABLE PersonsĪDD CONSTRAINT CHK_PersonAge CHECK (Age>=18 AND City='Sandnes') DROP a CHECK Constraint To create a CHECK constraint on the “Age” column when the table is already created, use the following SQL: MySQL / SQL Server / Oracle / MS Access: CREATE TABLE Persons (ĬONSTRAINT CHK_Person CHECK (Age>=18 AND City='Sandnes') To allow naming of a CHECK constraint, and for defining a CHECK constraint on multiple columns, use the following SQL syntax: SQL Server / Oracle / MS Access: CREATE TABLE Persons (
The CHECK constraint ensures that you can not have any person below 18 years: The following SQL creates a CHECK constraint on the “Age” column when the “Persons” table is created. If you define a CHECK constraint on a table it can limit the values in certain columns based on values in other columns in the row. If you define a CHECK constraint on a single column it allows only certain values for this column. The CHECK constraint is used to limit the value range that can be placed in a column. | 7 | Edgar Frank "Ted" | Codd | Edgar Frank "Ted" Codd | | 6 | Sophie | Freeman | Sophie Freeman | | 3 | Spencer | Pautier | Spencer Pautier | | 2 | Teri | Gutierrez | Teri Gutierrez | | studentID | FirstName | LastName | FullName | select studentID, FirstName, LastName, FirstName + ' ' + LastName as FullNameįrom student +-+-+-+-+ We can combine the First and the Last name columns to create the FullName calculated column. The database stores the studentID, FirstName, and LastName of the student. This example shows three columns selected from the “student” table and one calculated column. There are also options you can apply to show data that is not a table column. The SELECT part of a query is normally to determine which columns of the data to show in the results. Some common SQL statements and queries The SQL Select Statement Select and From clauses We also recommend Harvard CS50's course on databases and SQL.Īnd if you're feeling up for it, here's an entire 9-hour tutorial on relational database design so you can build your own RDBMS system using SQL. We recommend starting with freeCodeCamp's 4 hour SQL database tutorial. It introduced the concept of accessing many records with one single command, using SQL Queries. SQL is commonly pronounced “sequel.” Its most popular variants are MySQL, PostgreSQL, and SQLite - a version of SQL which is commonly used for prototyping. It is the most common tool used to manipulate and manage data in a relational database (often referred to as a “SQL database”). SQL stands for Structured Query Language.