Change 'what' to 'that' (#28)

* Change 'what' to 'that'

* More typo fixes

Author: username1001

Instructions/Labs/01-get-started-with-tsql.md

@@ -167,9 +167,9 @@ We've seen some examples of queries that return *NULL* values. *NULL* is general
     FROM SalesLT.Product;
     ```
 
-    The previous query returns the last date on which the product selling status was updated, but doesn't actually tell us the sales status itself. To determine that, we'll need to check the dates to see if the **SellEndDate** is *NULL*. To do this, you can use a **CASE** expression in the **SELECT** clause to check for *NULL* **SellEndDate** values. The **CASE** expression has two variants: a *simple* **CASE** what evaluates a specific column or value, or a *searched* **CASE** that evaluates one or more expressions.
+    The previous query returns the last date on which the product selling status was updated, but doesn't actually tell us the sales status itself. To determine that, we'll need to check the dates to see if the **SellEndDate** is *NULL*. To do this, you can use a **CASE** expression in the **SELECT** clause to check for *NULL* **SellEndDate** values. The **CASE** expression has two variants: a *simple* **CASE** that evaluates a specific column or value, or a *searched* **CASE** that evaluates one or more expressions.
 
-    In this example, or **CASE** experssion must determine if the **SellEndDate** column is *NULL*. Typically, when you are trying to check the value of a column you can use the **=** operator; for example the predicate **SellEndDate = '01/01/2005'** returns **True** if the **SellEndDate** value is *01/01/2005*, and **False** otherwise. However, when dealing with *NULL* values, the default behavior may not be what you expect. Remember that *NULL* actually means *unknown*, so using the **=** operator to compare two unknown values always results in a value of *NULL* - semantically, it's impossible to know if one unknown value is the same as another. To check to see if a value is *NULL*, you must use the **IS NULL** predicate; and conversely to check that a value is not *NULL* you can use the **IS NOT NULL** predicate.
+    In this example, our **CASE** expression must determine if the **SellEndDate** column is *NULL*. Typically, when you are trying to check the value of a column you can use the **=** operator; for example the predicate **SellEndDate = '01/01/2005'** returns **True** if the **SellEndDate** value is *01/01/2005*, and **False** otherwise. However, when dealing with *NULL* values, the default behavior may not be what you expect. Remember that *NULL* actually means *unknown*, so using the **=** operator to compare two unknown values always results in a value of *NULL* - semantically, it's impossible to know if one unknown value is the same as another. To check to see if a value is *NULL*, you must use the **IS NULL** predicate; and conversely to check that a value is not *NULL* you can use the **IS NOT NULL** predicate.
 
 6. Run the following query, which includes *searched* **CASE** that uses an **IS NULL** expression to check for *NULL* **SellEndDate** values.