From 87bb85ebcc5a7f4548d6f3528b89602f3883062b Mon Sep 17 00:00:00 2001
From: thomasabishop <thomas.actonbishop@bbc.co.uk>
Date: Tue, 10 Jan 2023 13:45:04 +0000
Subject: [PATCH] More SQL notes

---
 .../Accessing_metadata_about_SQL_database.md  |  7 ++-
 .../Autoincrement_for_unique_key_in_SQL.md    |  8 +++
 Databases/SQL/Data_types_in_MySQL.md          | 13 ++---
 Databases/SQL/Useful_operators_in_SQL.md      | 49 +++++++++++++++++++
 4 files changed, 69 insertions(+), 8 deletions(-)
 create mode 100644 Databases/SQL/Autoincrement_for_unique_key_in_SQL.md
 create mode 100644 Databases/SQL/Useful_operators_in_SQL.md

diff --git a/Databases/SQL/Accessing_metadata_about_SQL_database.md b/Databases/SQL/Accessing_metadata_about_SQL_database.md
index cc8c30e..69649b7 100644
--- a/Databases/SQL/Accessing_metadata_about_SQL_database.md
+++ b/Databases/SQL/Accessing_metadata_about_SQL_database.md
@@ -10,9 +10,12 @@ tags: [SQL]
 ```sql
 SHOW DATABASES
 
-USE [database_name]
-
 SHOW TABLES
 
+-- View column names and data types
 DESCRIBE [table_name]
+
+-- View disk usage per column
+SHOW TABLE STATUS
+
 ```
diff --git a/Databases/SQL/Autoincrement_for_unique_key_in_SQL.md b/Databases/SQL/Autoincrement_for_unique_key_in_SQL.md
new file mode 100644
index 0000000..18621e9
--- /dev/null
+++ b/Databases/SQL/Autoincrement_for_unique_key_in_SQL.md
@@ -0,0 +1,8 @@
+---
+categories:
+  - Databases
+  - Programming Languages
+tags: [SQL, relational-database]
+---
+
+# Autoincrement and `SERIAL`
diff --git a/Databases/SQL/Data_types_in_MySQL.md b/Databases/SQL/Data_types_in_MySQL.md
index 21c68ca..026b787 100644
--- a/Databases/SQL/Data_types_in_MySQL.md
+++ b/Databases/SQL/Data_types_in_MySQL.md
@@ -13,15 +13,16 @@ An understanding of the differences between data types is important because it o
 
 ### Integer (`INT`)
 
-No fractional part, fixed number of digits
+- No fractional part, fixed number of digits
+- `TINYINT`, `BIGINT` etc are all instances of `INT`, the difference is the size of the number stored.
 
-### Fixed point
+### Fixed point (`DECIMAL`)
 
-Contains decimal point. Use when accuracy is more important that representing very large or very small values
+Contains decimal point. Use when accuracy is more important that representing very large or very small values. Hence you would use this for monetary amounts.
 
-### Floating point
+### Floating point (`FLOAT`)
 
-Contains decimal point. Use when the ability to represent very large and very small values is more important than precision
+Contains decimal point. Use when the ability to represent very large and very small values is more important than precision. Hence you would not use this for monetary amounts.
 
 ## String
 
@@ -29,7 +30,7 @@ Contains decimal point. Use when the ability to represent very large and very sm
 
 Spaces are stripped in storage but represented with a character set.
 
-An example would be `CHAR(10)` or `CHAR(3)`. Here we set the upper limit but it must be the case that no string exceeds it.
+An example would be `CHAR(10)` or `CHAR(3)`. Here we set the upper limit but it must be the case that no string exceeds it. For either of these, if you add a value that is, e.g, two characters in length, it will add spaces to pad it out and make up the upper limit.
 
 ### Variable-length character strings
 
diff --git a/Databases/SQL/Useful_operators_in_SQL.md b/Databases/SQL/Useful_operators_in_SQL.md
new file mode 100644
index 0000000..323e8c1
--- /dev/null
+++ b/Databases/SQL/Useful_operators_in_SQL.md
@@ -0,0 +1,49 @@
+---
+categories:
+  - Databases
+  - Programming Languages
+tags: [SQL, relational-database]
+---
+
+# Useful operators in SQL
+
+## True and false
+
+> In SQL false results are represented by `0` and true results are represented by `1`. Any zero value is false and any non-zero value is true.
+
+> When comparing operators we can use `SELECT` as a general "execute" function for SQL to interpret
+
+```sql
+SELECT 0 = 0 -- 1
+SELECT 0.0 = 0 -- 1
+SELECT 9 >= 12 -- 0
+```
+
+## Boolean operators
+
+SQL supports all Boolean operators:
+
+```sql
+SELECT (9 > 7) AND (3 < 5) -- 1
+SELECT (3 > 1) OR (2 > 5) -- 1
+SELECT (3 > 1) XOR (5 > 1) -- 0
+```
+
+We also have `IS` and `IS NOT` for testing Boolean values:
+
+```sql
+SELECT (9 > 7) IS TRUE -- 1
+SELECT (9 > 7) IS NOT TRUE -- 0
+```
+
+## `NULL`
+
+- `NULL` is neither true (1) or false (0). It has no value.
+- An empty string (`""`) is **not** `NULL`
+
+```sql
+SELECT 1 IS NULL -- 0
+SELECT 1 IS NOT NULL -- 1
+SELECT NULL IS NULL -- 1
+SELECT "" IS NULL -- 0
+```