Java – H2: how to tell if table exists

h2javajdbcsql

I'm trying to write Java code that checks to see if an H2 table exists: if it doesn't exist, it first executes a CREATE TABLE query that creates the table before it proceeds any further.

I followed the advice in this Google Groups question but it simply does not work.

If I run the following query:

SELECT COUNT(*) AS count FROM information_schema.tables WHERE table_name = 'word_types'

I get back a single row with a COUNT field which has a value of 0; this indicates that the word_types tables doesn't exist. But when I run:

SELECT * FROM word_types

I get back 0 result sets, but the SQL frontend/GUI that I'm using shows me all the fields/columns that exist in the word_types table. Additionally, when I drill down into my database's list of available tables (using the same GUI), I see word_types exists.

So what is the correct query to use when trying to determine if an H2 table exists or not? Using v1.3.173. Thanks in advance!

Best Answer

First: check the case in which you type tables' names. It's very important. word_types and WORD_TYPES are two different tables.
Second: If you want to check if table exists and if it doesn't then create one, I recommend you to use the following example:

CREATE TABLE IF NOT EXISTS TEST(ID INT PRIMARY KEY, NAME VARCHAR(255));

Syntax:

ALTER TABLE {TABLENAME} 
ADD {COLUMNNAME} {TYPE} {NULL|NOT NULL} 
CONSTRAINT {CONSTRAINT_NAME} DEFAULT {DEFAULT_VALUE}
WITH VALUES

Example:

ALTER TABLE SomeTable
        ADD SomeCol Bit NULL --Or NOT NULL.
 CONSTRAINT D_SomeTable_SomeCol --When Omitted a Default-Constraint Name is autogenerated.
    DEFAULT (0)--Optional Default-Constraint.
WITH VALUES --Add if Column is Nullable and you want the Default Value for Existing Records.

Notes:

Optional Constraint Name:
If you leave out CONSTRAINT D_SomeTable_SomeCol then SQL Server will autogenerate
a Default-Contraint with a funny Name like: DF__SomeTa__SomeC__4FB7FEF6

Optional With-Values Statement:
The WITH VALUES is only needed when your Column is Nullable
and you want the Default Value used for Existing Records.
If your Column is NOT NULL, then it will automatically use the Default Value
for all Existing Records, whether you specify WITH VALUES or not.

How Inserts work with a Default-Constraint:
If you insert a Record into SomeTable and do not Specify SomeCol's value, then it will Default to 0.
If you insert a Record and Specify SomeCol's value as NULL (and your column allows nulls),
then the Default-Constraint will not be used and NULL will be inserted as the Value.

Notes were based on everyone's great feedback below.
Special Thanks to:
@Yatrix, @WalterStabosz, @YahooSerious, and @StackMan for their Comments.

Best Answer

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Syntax:

Example:

Notes:

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